feat: Pipeline Settings M4 — generation history list + per-step I/O drill-down (v0.26.0)

CEO ask #5 complete: browsable paginated run list + full input/output inspection
per pipeline stage.

## What was built

### Generations list (prautoblogger-gen-history)
Hidden admin page (options.php-parent, priority 14) showing all pipeline runs newest
first, 20 per page. Each row: article title (linked to dossier when a post exists),
start date, status chip, distinct model(s), settled cost, duration. Queries bounded
(LIMIT/OFFSET) — no unbounded SELECT *.

New files:
- includes/admin/class-gen-history-query.php — paginated run list + per-run I/O queries
- includes/admin/class-gen-history-page.php  — hidden admin page + asset enqueue
- assets/css/gen-history.css, assets/js/gen-history.js — table + drill-down UI
- templates/admin/gen-history-page.php       — HTML template

### Per-step input + output drill-down (Stage I/O)
'Stage I/O' toggle opens an AJAX-loaded inline panel. For each generation_log stage:
  - input_system: system-role message content from generation_log.request_json
  - input_user: last user-role message from request_json (rendered instruction)
  - output: model response from run_stages.meta_json.output
  - output_pruned: true when meta_json present but output key absent (retention prune)

New AJAX endpoint: prautoblogger_gen_run_io (manage_options + nonce)
New file: includes/ajax/class-gen-run-io-handler.php

### gen-log INPUT/OUTPUT coverage verified
- INPUT: generation_log.request_json stores the chat body WITHOUT the Authorization
  header (Request_Recorder captures body after build_body(), before headers are added).
  System + user message content extracted and surfaced.
- OUTPUT: run_stages.meta_json.output is the LLM response text (Run_Stage_State::done()).
  Image/log-only stages (image_a, image_b, llm_research, image_prompt_rewrite) have no
  run_stages row; output is null, NOT pruned. Pruned outputs are flagged honestly.

### Dossier as entry point
Runs with a linked post show a 'Dossier' button deep-linking into the existing dossier
(which renders full per-stage I/O with editorial context). Stage I/O is complementary —
works for failed/orphan runs and provides the same data in a lighter inline UI.

### Stage-list-driven
Stage labels via Stage_Display_Map::label() — Phase 2b curate/seo appear automatically.

### Dead prautoblogger_pipeline_history endpoint
Not dead — it IS used by pipeline-settings.js for prompt version history (M3 feature).
Left in place. No dangling endpoint.

## M3 P2 sweep
- Removed dead data-preview-nonce and data-diff-nonce HTML attrs from
  pipeline-settings-prompt-panel.php (JS uses prabPipeline.*Nonce from
  wp_localize_script, not data-* attrs — confirmed by grep).
- Removed 3 redundant wp_create_nonce() calls from class-pipeline-settings-renderer.php
  (preview_nonce, history_nonce, diff_nonce keys that fed the now-removed dead attrs).
- Updated CONTEXT.md: added M3 + M4 terminology tables.
- Updated ARCHITECTURE.md: added ajax/ section to file tree; M3/M4 design notes.
- Updated INFORMATION-ARCHITECTURE.md: prautoblogger-gen-history entry.

## Security
- manage_options cap + prautoblogger_gen_run_io nonce on the AJAX endpoint.
- All output esc_html()/esc_url() server-side before JSON-encoding.
- JS renders prompt/response text via textContent — never innerHTML.
- No writes from history UI; request_json never contains auth headers.

## Version: v0.25.0 → v0.26.0

Agent-Session: pipeline-settings-m4

CONTEXT.md

@@ -119,3 +119,30 @@ Full table names are defined in `class-activator.php`.
 | **`step_context`** | The POST key that routes `save_step_settings` to the correct field set. Expected values: `global`, `research`, `analysis`, `writer`, `editorial`. Validated with `sanitize_key()` before use. |
 | **`pipeline_action=save_step_settings`** | POST contract key introduced in M2. Triggers `PRAutoBlogger_Pipeline_Settings_Save_Handler::handle_step_settings_save()`. Parallel to the existing `save_model`, `save_prompt`, and `reset_prompt` action values. |
 | **relocated-tabs concept** | The Settings tabs `prautoblogger_models`, `prautoblogger_content`, and `prautoblogger_sources` were retired in M2; their 17 option fields moved to per-step panels in the Pipeline page. The `wp_option` keys are unchanged — only the admin UI surface that edits them moved. See `CONVENTIONS.md §Retired Settings Tabs`. |
+
+---
+
+## Pipeline Settings M3 (v0.25.0)
+
+| Term | Definition |
+|------|-----------|
+| **assembled instructions** | The fully-rendered text the LLM actually received for a given prompt key — all `{{ token }}` placeholders substituted with live data. Surfaced in the Template/Preview toggle on the Pipeline Settings page. Preferred source: last successful `generation_log.request_json` row for the stage; fallback: sample render with `[token_name]` placeholders. |
+| **Template mode** | The default view of a prompt panel: shows the editable textarea with raw template text (including `{{ token }}` placeholders). Editing here writes a new version to the prompt registry. |
+| **Preview mode** | Read-only view of a prompt panel: shows the assembled instructions the LLM received in the last run (or a sample render if no run exists). No save path in PHP or JS. |
+| **Template-vs-Preview guardrail** | The Template and Preview panes are mutually exclusive. Preview has no save path server-side (no write handler) or client-side (JS sends no save POST when in preview mode). Prevents accidental edits to the rendered output. |
+| **version diff** | An LCS-based line-level diff between two prompt versions (or a version and the factory default). Rendered server-side as `{type, text}` lines (added/removed/context/omitted); the JS applies colours without parsing unified-diff format. Context window: 3 lines before/after each changed block. |
+
+---
+
+## Pipeline Settings M4 (v0.26.0)
+
+| Term | Definition |
+|------|-----------|
+| **Generation History** | The `prautoblogger-gen-history` admin page: paginated list of all pipeline runs, newest first. Entry point to the per-step I/O drill-down. |
+| **run row** | A single entry in the Generation History list. Corresponds to one `prautoblogger_runs` ledger row. Shows title (from linked post), date, status chip, models used, cost, duration. |
+| **Stage I/O** | The inline drill-down panel for a run: shows every generation_log stage's full INPUT (system + user message content from `request_json`) and OUTPUT (model response from `run_stages.meta_json.output`). Loaded via AJAX on first toggle; cached client-side for subsequent toggles. |
+| **input_system** | The `system`-role message content extracted from `generation_log.request_json`. The prompt template the LLM received. May be null for image/non-chat stages. |
+| **input_user** | The last `user`-role message content from `request_json`. The rendered, token-filled instruction the LLM received. |
+| **output** | The LLM's raw response text from `run_stages.meta_json.output`. Null when the stage is log-only (image_a, image_b, llm_research, image_prompt_rewrite — no run_stages row). |
+| **output_pruned** | True when `run_stages.meta_json` exists but has no `output` key — indicating the output was pruned by the `prautoblogger_request_json_retention_days` setting. The UI shows an explicit message instead of a blank field. |
+| **log-only stage** | A generation_log stage with no corresponding run_stages row. Image stages (image_a, image_b, image_prompt_rewrite) and llm_research fall here. Their output is null (not pruned) in the drill-down. |