feat(prompts): versioned prompt registry, v1 seed, pinning + stamping

Pipeline v2 Phase 1, commit 2/4. Prompt_Registry (read) +
Prompt_Registry_Writer (immutable create/activate/seed) over the new
prompts table; canonical v1 bodies extracted verbatim into
Prompt_Defaults(+Editorial) — single source for BOTH the seed migration
(Migrate_Prompt_Seed_V0180, flag-gated, self-healing) and the in-code
fallback, so a missing table renders byte-identical prompts and never
fatals. Call sites swapped: Content_Prompts, Analysis_Prompts,
Chief_Editor, LLM_Research_Provider. Includes the image-composer seam
keys image.rewriter_system + image.style_template (composer PR consumes,
must not add its own prompt storage).

Pinning: Cost_Tracker::set_run_id() registers the process run context,
ensures the runs ledger row, and pins active versions once per run;
log_api_call/log_image_generation stamp prompt_version (run pins) +
agent_role (stage map) on every generation_log row. Single-pass keeps
stage 'draft' and stamps content.single_pass explicitly.

Under-300 refactors: Content_Generator's four duplicated span/log blocks
consolidated into execute_stage() (301 -> 224 lines, behavior
identical); get_avg_tokens_for_stages body moved to Cost_Reporter with a
delegating wrapper kept for API compatibility.

Tests: byte-identical fallback renders (single_pass/outline/polish/
editor.system), token filling, key coverage incl. stage-map integrity.

Agent-Session: pipeline-v2-phase1-engineer-20260612

CONVENTIONS.md

@@ -320,6 +320,38 @@ The field renderer, AJAX refresh, model picker JS, and capability filtering are
 
 Phase 3 will add provider selection; v0.11.0 is OpenRouter-only.
 
+## How To: Change a Pipeline Prompt (v0.18.0+)
+
+Prompt copy is **versioned data**, not code. The bodies live in
+`wp_prautoblogger_prompts` (one ACTIVE version per key); the hardcoded texts in
+`class-prompt-defaults.php` / `class-prompt-defaults-editorial.php` are the canonical v1 seed
+AND the fallback when the table is unavailable — never edit a stored version in place.
+
+1. **Never `UPDATE` a body.** Create a new version and activate it:
+   ```php
+   $v = PRAutoBlogger_Prompt_Registry_Writer::create_version(
+       'content.polish', $new_body, $model, $params, 'your-name', true
+   );
+   ```
+2. **Keys** are dot-namespaced: `content.system`, `content.single_pass`, `content.outline`,
+   `content.draft`, `content.polish`, `analysis.system`, `analysis.user`, `editor.system`,
+   `editor.review`, `research.system`, `image.rewriter_system`, `image.style_template`.
+   The two `image.*` keys are the image-composer seam — coordinate on the convo thread
+   before touching them.
+3. **Tokens** use the `{{ name }}` syntax (exactly one space each side — same convention as
+   the Style Template's `{{ topic_summary }}`). Token VALUES are computed by code
+   (`Content_Prompts`, `Analysis_Prompts`, `Chief_Editor`); a new prompt version may reorder
+   or drop tokens but must not invent new ones without adding the value at the call site.
+4. **Pinning.** A run pins the active version of every key at start
+   (`runs.pinned_prompts_json`, written by `Cost_Tracker::set_run_id()`); every
+   generation_log row is stamped with the pinned `prompt_version`. Activating a new version
+   affects the NEXT run, never a run in flight.
+5. **Call sites** render via `PRAutoBlogger_Prompt_Registry::render( $key, $tokens )` and
+   must keep working when the table is absent (the registry falls back to the defaults
+   automatically — do not add your own fallback).
+6. If you add a key: define the body in the appropriate defaults class, add it to `defs()`,
+   map its stage in `Stage_Display_Map`, and it will seed on the next migration pass.
+
 ## Git Workflow — PR-Gated, Soft-Enforced
 
 This repo is private on GitHub's free plan, which does not support branch protection or rulesets. The review gate is enforced at the agent layer, not server-side. Every agent and human contributor follows these rules; the `.github/workflows/main-push-audit.yml` tripwire opens an audit issue on any direct push to `main` that did not come from a merged PR.