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/admin/fields/class-open-router-model-field.php

@@ -141,6 +141,8 @@ class PRAutoBlogger_OpenRouter_Model_Field {
 			'prautoblogger_analysis_model' => array( 'analysis' ),
 			'prautoblogger_writing_model'  => array( 'outline', 'draft', 'polish' ),
 			'prautoblogger_editor_model'   => array( 'review' ),
+			'prautoblogger_research_model' => array( 'research', 'llm_research' ),
+			'prautoblogger_image_model'    => array( 'image_a', 'image_b' ),
 		);
 		return $map[ $field_id ] ?? array();
 	}