feat(fork-config): add automated fork configuration with npm run configure

Add a complete fork configuration system that allows users to set up their
forked site with a single command or follow manual instructions.

## New files

- FORK_CONFIG.md: Comprehensive guide with two setup options
  - Option 1: Automated JSON config + npm run configure
  - Option 2: Manual step-by-step instructions with code snippets
  - AI agent prompt for automated updates

- fork-config.json.example: JSON template with all configuration fields
  - Site info (name, title, description, URL, domain)
  - GitHub and contact details
  - Creator section for footer links
  - Optional feature toggles (logo gallery, GitHub graph, blog page)
  - Theme selection

- scripts/configure-fork.ts: Automated configuration script
  - Reads fork-config.json and applies changes to all files
  - Updates 11 configuration files in one command
  - Type-safe with ForkConfig interface
  - Detailed console output showing each file updated

## Updated files

- package.json: Added configure script (npm run configure)
- .gitignore: Added fork-config.json to keep user config local
- files.md: Added new fork configuration files
- changelog.md: Added v1.18.0 entry
- changelog-page.md: Added v1.18.0 section with full details
- TASK.md: Updated status and completed tasks
- README.md: Replaced Files to Update section with Fork Configuration
- content/blog/setup-guide.md: Added Fork Configuration Options section
- content/pages/docs.md: Added Fork Configuration section
- content/pages/about.md: Added fork configuration mention
- content/blog/fork-configuration-guide.md: New featured blog post

## Files updated by configure script

| File                                | What it updates                        |
| ----------------------------------- | -------------------------------------- |
| src/config/siteConfig.ts            | Site name, bio, GitHub, features       |
| src/pages/Home.tsx                  | Intro paragraph, footer links          |
| src/pages/Post.tsx                  | SITE_URL, SITE_NAME constants          |
| convex/http.ts                      | SITE_URL, SITE_NAME constants          |
| convex/rss.ts                       | SITE_URL, SITE_TITLE, SITE_DESCRIPTION |
| index.html                          | Meta tags, JSON-LD, page title         |
| public/llms.txt                     | Site info, GitHub link                 |
| public/robots.txt                   | Sitemap URL                            |
| public/openapi.yaml                 | Server URL, site name                  |
| public/.well-known/ai-plugin.json   | Plugin metadata                        |
| src/context/ThemeContext.tsx        | Default theme                          |

## Usage

Automated:
  cp fork-config.json.example fork-config.json
  # Edit fork-config.json
  npm run configure

Manual:
  Follow FORK_CONFIG.md step-by-step guide

fork-config.json.example

@@ -0,0 +1,44 @@
+{
+  "siteName": "Your Site Name",
+  "siteTitle": "Your Tagline",
+  "siteDescription": "A one-sentence description of your site.",
+  "siteUrl": "https://yoursite.netlify.app",
+  "siteDomain": "yoursite.netlify.app",
+  "githubUsername": "yourusername",
+  "githubRepo": "your-repo-name",
+  "contactEmail": "you@example.com",
+  "creator": {
+    "name": "Your Name",
+    "twitter": "https://x.com/yourhandle",
+    "linkedin": "https://www.linkedin.com/in/yourprofile/",
+    "github": "https://github.com/yourusername"
+  },
+  "bio": "Write markdown, sync from the terminal. Your content is instantly available to browsers, LLMs, and AI agents.",
+  "logoGallery": {
+    "enabled": true,
+    "title": "Built with",
+    "scrolling": false,
+    "maxItems": 4
+  },
+  "gitHubContributions": {
+    "enabled": true,
+    "showYearNavigation": true,
+    "linkToProfile": true,
+    "title": "GitHub Activity"
+  },
+  "blogPage": {
+    "enabled": true,
+    "showInNav": true,
+    "title": "Blog",
+    "description": "All posts from the blog, sorted by date.",
+    "order": 2
+  },
+  "postsDisplay": {
+    "showOnHome": true,
+    "showOnBlogPage": true
+  },
+  "featuredViewMode": "cards",
+  "showViewToggle": true,
+  "theme": "tan"
+}
+