feat: Pipeline Settings M1 — per-step model picker, system instructions, agent prompts (v0.23.0)

Adds a new 'Pipeline' wp-admin submenu page (prautoblogger-pipeline) with a
step rail covering all LLM stages: Research, Analysis, Writer, Editorial, Image.

Per-step panels surface:
- The existing PRAutoBlogger_OpenRouter_Model_Field picker (no new dropdown) with
  stage-accurate cost-preview for each step.
- System-instructions editor + agent-prompt editor(s) bound to the versioned
  prompt registry. Saving creates a new immutable version; reset-to-default
  available; active version number shown per panel.
- Step params read from PRAutoBlogger_Prompt_Registry::defs() (read-only in M1).

Writer panel marks inactive sub-stage prompts based on current pipeline mode.

No existing Settings sections changed — both surfaces edit the same wp_options
and prompts table. Decomposition of AI Models / Content / Sources tabs is M2.

get_stages_for_setting() extended: prautoblogger_research_model → [research,
llm_research] and prautoblogger_image_model → [image_a, image_b] for
cost-preview accuracy.

New files (4 classes + 1 CSS + 1 JS + 2 templates, all <=300 lines):
  includes/admin/class-pipeline-settings-page.php
  includes/admin/class-pipeline-settings-renderer.php
  includes/admin/class-pipeline-settings-save-handler.php
  includes/admin/class-pipeline-settings-step-map.php
  assets/css/pipeline-settings.css
  assets/js/pipeline-settings.js
  templates/admin/pipeline-settings-page.php
  templates/admin/pipeline-settings-prompt-panel.php

Modified:
  includes/admin/fields/class-open-router-model-field.php
  includes/class-prautoblogger.php
  prautoblogger.php (v0.23.0)
  ARCHITECTURE.md / INFORMATION-ARCHITECTURE.md / CHANGELOG.md

No new wp_options; uninstall unchanged.

Agent-Session: pipeline-settings-m1

includes/class-prautoblogger.php

@@ -72,6 +72,11 @@ class PRAutoBlogger {
 		add_action( 'admin_menu', array( $board_page, 'on_register_menu' ), 11 );
 		add_action( 'admin_enqueue_scripts', array( $board_page, 'on_enqueue_assets' ) );
 
+		// Pipeline Settings: visible submenu, priority 13 (after dossier).
+		$pipeline_page = new PRAutoBlogger_Pipeline_Settings_Page();
+		add_action( 'admin_menu', array( $pipeline_page, 'on_register_menu' ), 13 );
+		add_action( 'admin_enqueue_scripts', array( $pipeline_page, 'on_enqueue_assets' ) );
+
 		// Dossier: link-accessed hidden submenu, priority 12 (after board).
 		$dossier_page = new PRAutoBlogger_Dossier_Page();
 		add_action( 'admin_menu', array( $dossier_page, 'on_register_menu' ), 12 );