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

INFORMATION-ARCHITECTURE.md

@@ -12,8 +12,8 @@ Maintained per DoD v1.1.0 (same-PR update rule).
 | `prautoblogger-settings` | `PRAutoBlogger_Admin_Page` | `admin-page.php` | Main settings: API Keys, Schedule & Budget, Publishing, Analytics, Display, Images. AI Models / Content / Sources retired to Pipeline Settings in M2 (v0.24.0). |
 | `prautoblogger-board` | `PRAutoBlogger_Board_Page` | `board-page.php` | Kanban board: Generating / Failed / In Review / Published columns. New Article button (v0.21.0). |
 | `prautoblogger-pipeline` | `PRAutoBlogger_Pipeline_Settings_Page` | `pipeline-settings-page.php` | Per-step pipeline config: Global Content Context (niche), step option fields, model picker, system instructions, agent prompts, params (M1 v0.23.0; M2 v0.24.0 adds editable step options, retires AI Models/Content/Sources Settings tabs). M3 v0.25.0 adds Template/Preview toggle (assembled-instructions preview from last-run gen_log), version history accordion, and inline diff panel. Three new AJAX endpoints: `prautoblogger_pipeline_preview`, `prautoblogger_pipeline_history`, `prautoblogger_pipeline_diff` (all `manage_options` + nonce gated). |
-| `prautoblogger-articles` | `PRAutoBlogger_Articles_Page` | `articles-page.php` | Paginated list of all articles/runs |
-| `prautoblogger-dossier` | `PRAutoBlogger_Dossier_Page` | `dossier-page.php` | Per-article generation log + stage edit/re-run (M3) |
+| `prautoblogger-gen-history` | `PRAutoBlogger_Gen_History_Page` | `gen-history-page.php` | M4: Paginated list of all generation runs — title, date, status, models, cost, duration; Stage I/O toggle for inline per-step input/output drill-down (AJAX). Hidden options.php-parent page. `manage_options` + `prautoblogger_gen_run_io` nonce. |
+| `prautoblogger-dossier` | `PRAutoBlogger_Dossier_Page` | `dossier-page.php` | Per-article generation log + stage edit/re-run (M3). Preferred per-step I/O surface for runs with a linked post. |
 | `prautoblogger-ideas` | `PRAutoBlogger_Ideas_Browser` | `ideas-browser.php` | Analysis results browser with per-idea generation |
 | `prautoblogger-activity` | `PRAutoBlogger_Activity_Page` | `activity-page.php` | Cost + generation activity log |