feat: add AI Agent chat integration with Anthropic Claude API

Add AI writing assistant (Agent) powered by Anthropic Claude API. Agent can be enabled on Write page (replaces textarea) and optionally in RightSidebar on posts/pages via frontmatter.

Features:
- AIChatView component with per-page chat history
- Page content context support for AI responses
- Markdown rendering for AI responses
- User-friendly error handling for missing API keys
- System prompt configurable via Convex environment variables
- Anonymous session authentication using localStorage

Environment variables required:
- ANTHROPIC_API_KEY (required)
- CLAUDE_PROMPT_STYLE, CLAUDE_PROMPT_COMMUNITY, CLAUDE_PROMPT_RULES (optional split prompts)
- CLAUDE_SYSTEM_PROMPT (optional single prompt fallback)

Configuration:
- siteConfig.aiChat.enabledOnWritePage: Enable Agent toggle on /write page
- siteConfig.aiChat.enabledOnContent: Allow Agent on posts/pages via frontmatter
- Frontmatter aiChat: true (requires rightSidebar: true)

Updated files:
- src/components/AIChatView.tsx: AI chat interface component
- src/components/RightSidebar.tsx: Conditional Agent rendering
- src/pages/Write.tsx: Agent mode toggle (title changes to Agent)
- convex/aiChats.ts: Chat history queries and mutations
- convex/aiChatActions.ts: Claude API integration with error handling
- convex/schema.ts: aiChats table with indexes
- src/config/siteConfig.ts: AIChatConfig interface
- Documentation updated across all files

Documentation:
- files.md: Updated component descriptions
- changelog.md: Added v1.33.0 entry
- TASK.md: Marked AI chat tasks as completed
- README.md: Added AI Agent Chat section
- content/pages/docs.md: Added AI Agent chat documentation
- content/blog/setup-guide.md: Added AI Agent chat setup instructions
- public/raw/changelog.md: Added v1.33.0 entry

fork-config.json.example

@@ -50,6 +50,11 @@
   "featuredViewMode": "cards",
   "showViewToggle": true,
   "theme": "tan",
-  "fontFamily": "serif"
+  "fontFamily": "serif",
+  "homepage": {
+    "type": "default",
+    "slug": null,
+    "originalHomeRoute": "/home"
+  }
 }