feat(images): wire image pipeline into article flow (commit 1b) (#14)

Completes the image + A/B experiment workstream by integrating
image generation into the post publishing pipeline.

New classes:
- class-image-pipeline.php: Orchestrates A/B generation (article-driven
  Image A → featured; source-driven Image B → post meta). Each image
  independently fallible; article publishes even if both fail.
- class-image-prompt-builder.php: Synthesizes visual prompts from article
  title+content and Reddit source data. Appends CEO-locked 90s infomercial
  style suffix. Keeps prompts concise for FLUX.1 quality.
- class-image-media-sideloader.php: Downloads image bytes, creates temp
  file, sideloads via media_handle_sideload(), sets alt text and metadata,
  cleans up temp files.

Changes to existing files:
- Publisher.attach_generated_images(): Runs image pipeline post-creation,
  sets _thumbnail_id (Image A) and _prautoblogger_image_b_id (Image B).
  Errors logged but don't block post publication.
- Settings: prautoblogger_image_enabled (toggle, default off).

Tests:
- ImagePipelineTest.php: Orchestration, cost tracking, graceful failures.
- ImagePromptBuilderTest.php: Prompt generation, length limits.

Docs:
- ARCHITECTURE.md: New step 6b in data flow, new files in file tree.
- CHANGELOG.md: Commit 1a/1b breakdown.

Co-authored-by: peptiderepo <peptiderepo@users.noreply.github.com>
Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

ARCHITECTURE.md

@@ -65,6 +65,14 @@ PRAutoBlogger is a WordPress plugin that monitors social media (starting with Re
              │
              ▼
 ┌─────────────────────────┐
+│  6b. Image Pipeline     │  (commit 1b) Generates two A/B images:
+│  (optional, if enabled) │  - Image A: article-driven (featured)
+│                         │  - Image B: source-driven (post meta)
+│                         │  Sideloads to media library, logs costs
+└────────────┬────────────┘
+             │
+             ▼
+┌─────────────────────────┐
 │  7. Metrics Collector   │  Tracks post-publish performance:
 │  (Separate cron job)    │  - WordPress native (views, comments)
 │                         │  - GA4 (pageviews, bounce rate, time on page)
@@ -119,6 +127,9 @@ prautoblogger/
 │   │   ├── class-content-generator.php# Writer agent — manages the generation pipeline
 │   │   ├── class-chief-editor.php     # Editor agent — LLM-powered editorial review
 │   │   ├── class-publisher.php        # Creates WordPress posts from approved content
+│   │   ├── class-image-pipeline.php   # Orchestrates A/B image generation (commit 1b)
+│   │   ├── class-image-prompt-builder.php # Generates visual prompts from article/source data
+│   │   ├── class-image-media-sideloader.php # Imports images into WordPress media library
 │   │   ├── class-cost-tracker.php     # Logs all API costs, enforces budget limits
 │   │   ├── class-logger.php           # Structured logging singleton (error/warning/info/debug)
 │   │   ├── class-pipeline-runner.php  # Orchestrates the full generation pipeline