docs: add app-local ADR log + ADR-0001 editorial illustration image style (#169)

Adds docs/adr/ scaffold (README index) and ADR-0001 documenting the v0.16.0 pivot to text-free editorial illustration with caption-as-HTML.

Agent-Session: adr-scaffold-20260617

docs/adr/0001-editorial-illustration-image-style.md

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+# ADR-0001: Editorial illustration image style — pivot from comic to text-free illustration with caption-as-HTML
+
+- **Status:** Accepted
+- **Date:** 2026-05-29
+- **Deciders:** CTO, Engineer PRAutoBlogger
+- **Supersedes:** —
+- **Superseded by:** —
+- **Related:** ARCHITECTURE.md §#20; CHANGELOG.md [0.16.0]; PR #146 (d00e746);
+  `docs/proposals/2026-05-29-image-pipeline-in-plugin-brief.md`;
+  ARCHITECTURE.md §#21 (v0.17.0 deterministic composer, the follow-on commit)
+
+---
+
+## Context
+
+PRAutoBlogger generates two AI images (A/B pair) for each article and publishes
+them as featured images. From the initial comic-strip launch (commit b17ed47,
+`feat: switch to single-panel newspaper comic style for article images`) through
+v0.15.x, the image style was a **single-panel newspaper comic** — the rewriter
+LLM was asked to produce a comic gag or visual pun, and the result was appended
+to a style suffix and sent to the diffusion model.
+
+By April 2026 two problems had accumulated:
+
+1. **Text legibility.** Small diffusion models (FLUX.1 schnell is the default
+   since v0.9.0 at ~$0.0006/image) cannot reliably render readable text inside
+   an image. Comic panels depend on legible speech bubbles or captions burned
+   into the image; both were routinely garbled (confirmed in the 2026-04-21 A/B
+   finding documented in ARCHITECTURE.md §#21).
+
+2. **Caption path already existed.** A separate HTML caption below the image had
+   been extracted as early as commit 214f57b (`Separate comic captions from
+   generated images`, 2026-04-18). The scene/caption split in
+   `Image_Scene_Parser` was already live, meaning there was an established path
+   for readable text that did not depend on the diffusion model rendering it.
+
+Together these created pressure to stop asking the diffusion model to render
+text at all, and instead shift all readable copy to the HTML caption path that
+was already in place.
+
+## Decision drivers
+
+1. **Image quality on default model.** FLUX.1 schnell (the default since v0.9.0)
+   produces garbled text in generated images. A style that depends on readable
+   in-image text degrades silently on the model we actually ship.
+2. **Editorial trust signal.** Peptide Repo's editorial direction is
+   science-informational; comic gags undercut the authority the site needs for
+   health-adjacent content.
+3. **Reuse the existing caption path.** The scene/caption split and HTML caption
+   path were already live; the change is a prompt rewrite + template substitution
+   with no new infrastructure.
+4. **Admin configurability.** The style suffix was a single string appended to
+   every prompt; a template with a named substitution token (`{{ topic_summary
+   }}`) is more maintainable and admin-overridable without a code deploy.
+5. **Cost and provider neutrality.** The change is prompt-only; provider, model,
+   and pricing are unchanged.
+
+## Options considered
+
+### Option A — Keep the comic style, fix text rendering per-model
+
+Ask the rewriter LLM to omit text from comic panels (no speech bubbles, no
+caption burns), and describe a visual scene instead. Keep the style suffix
+pattern.
+
+**Pros:**
+- Minimal code change — just a prompt edit.
+- Comic aesthetic is recognisable and differentiating.
+
+**Cons:**
+- Does not solve the underlying mismatch: the style suffix is a static string
+  that gives no per-article context to the diffusion model. The rewriter scene
+  still has no slot to insert topic-specific imagery.
+- Comic gags require the LLM to invent a joke per peptide, which is inconsistent
+  in tone.
+- Comic aesthetic conflicts with the editorial trust direction for health content.
+
+### Option B — Pivot to text-free editorial scientific illustration + template substitution (chosen)
+
+Change the rewriter system prompt so the LLM emits a concise 1-2 sentence
+topic/mechanism summary as the SCENE — one concrete centered focal subject, no
+text, no people-as-gag, no logos. Replace the style suffix with an editable
+template (`prautoblogger_image_style_template`) that contains a single
+`{{ topic_summary }}` token filled per-article by
+`PRAutoBlogger_Image_Template_Filler`. The CAPTION line from the scene/caption
+split continues to flow to the HTML-caption-below-image path unchanged.
+
+**Pros:**
+- Diffusion model is never asked to render text; garbled-text failures are
+  eliminated.
+- The editorial scientific illustration style is consistent with the site's
+  authority positioning.
+- Template substitution gives admins a single editable field to control the
+  entire visual style without touching the per-article rewriter logic.
+- Degradation is explicit: missing token → append + warning; empty summary →
+  style-only prompt; a blank prompt is never sent to the provider.
+- Provider, model, and cost are unchanged.
+
+**Cons:**
+- Breaks the comic gag aesthetic for any site running PRAutoBlogger in a
+  context where that was intentional (low risk for this deployment).
+- The old `prautoblogger_image_style_suffix` option must be migrated
+  (handled via one-time migration `prautoblogger_migrated_style_template_v0160`
+  and mirroring to `_deprecated` for one cycle).
+
+### Option C — Generate text overlays in PHP (deterministic composer only)
+
+Skip the style change; solve legibility by compositing text onto images in PHP
+after the diffusion model runs.
+
+**Pros:**
+- Diffusion model input is unchanged (comic style preserved).
+- Composited text is always readable regardless of model.
+
+**Cons:**
+- Compositing a comic-style gag is much harder than compositing a caption band
+  (arbitrary layout vs. a fixed panel at the bottom).
+- Does not address the editorial direction mismatch.
+- The deterministic PHP composer (ARCHITECTURE.md §#21) was already planned as
+  a follow-on commit specifically for OG/social variants — pursuing it as the
+  sole fix conflates two separate problems.
+
+## Decision
+
+**Option B** — pivot to text-free editorial scientific illustration with
+caption-as-HTML and template substitution.
+
+This was shipped as v0.16.0 (2026-05-29, PR #146, commit d00e746). The changes
+are entirely in the prompt layer:
+
+- `Image_Prompt_Builder::REWRITER_SYSTEM_PROMPT` now asks for a concise
+  topic/mechanism SCENE and keeps the CAPTION line for the HTML path.
+- `PRAutoBlogger_Image_Template_Filler::fill()` replaces the old
+  `trim($scene . ' ' . $style_suffix)` concat at all three entry points
+  (`build_article_prompt`, `build_source_prompt`, `build_fallback_prompt`).
+- New admin setting `prautoblogger_image_style_template` with
+  single-token validation replaces the Style Suffix field.
+- One-time migration preserves the old suffix value for one cycle.
+
+The deterministic PHP composer for OG/social branding (ARCHITECTURE.md §#21)
+was deliberately deferred to a follow-on commit (v0.17.0, #152) so that the
+prompt-only change could ship and be verified first.
+
+## Consequences
+
+**Accepted benefits:**
+- Garbled-text image failures are eliminated on the default FLUX.1 schnell model.
+- The editorial scientific illustration style is consistent with the site's
+  health-content positioning and sustainable across future model changes.
+- The template + token pattern makes style updates admin-configurable without
+  code changes.
+
+**Accepted costs:**
+- The comic aesthetic is gone and cannot be restored by toggling a setting;
+  switching back requires a prompt and template change.
+- The `prautoblogger_image_style_suffix` option is deprecated; the migration
+  runs once on upgrade and the old option is not deleted until a future cycle.
+
+**Follow-up created:**
+- ARCHITECTURE.md §#21 (v0.17.0): deterministic PHP image composer to composite
+  branded OG and square card variants — necessary because the social/OG image
+  travels without the HTML caption and therefore needs text burned in by PHP,
+  not by the diffusion model.

docs/adr/README.md

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+# PRAutoBlogger — App-Local ADR Log
+
+This folder is the per-repo decision log for the **PRAutoBlogger** plugin.
+It records architectural choices that affect this codebase alone.
+
+**Cross-cutting decisions** (spanning multiple plugins, shared infra, or
+CI/deploy standards) live centrally in
+`Peptide Repo CTO/docs/engineering/decisions/`.
+
+For the routing rule, required sections, and status lifecycle, read the
+canonical process document:
+`Peptide Repo CTO/docs/engineering/ADR-PROCESS.md`
+
+The canonical template for new ADRs is:
+`Peptide Repo CTO/docs/engineering/adr-template.md`
+
+## Numbering
+
+ADRs in this folder use a per-repo zero-padded sequence starting at `0001`.
+A new ADR always gets the next unused number; numbers are never reused.
+
+## Index
+
+| Number | Title | Status | Date |
+|--------|-------|--------|------|
+| [0001](0001-editorial-illustration-image-style.md) | Editorial illustration image style — pivot from comic to text-free illustration with caption-as-HTML | Accepted | 2026-05-29 |