Add post migration and author filter features

- Add migrate command to transfer posts between websites
- Support CSV-based and filtered migration modes
- Preserve original post dates (with --ignore-original-date option)
- Auto-create categories and tags on destination site
- Add author filtering to export (--author and --author-id flags)
- Include author_name column in exported CSV
- Add comprehensive documentation (MIGRATION_GUIDE.md, AUTHOR_FILTER_GUIDE.md)

Co-authored-by: Qwen-Coder <qwen-coder@alibabacloud.com>

AUTHOR_FILTER_GUIDE.md

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+# Author Filter Guide
+
+Export posts from specific authors using the enhanced export functionality.
+
+## Overview
+
+The export command now supports filtering posts by author name or author ID, making it easy to:
+- Export posts from a specific author across all sites
+- Combine author filtering with site filtering
+- Export posts from multiple authors at once
+
+## Usage
+
+### Filter by Author Name
+
+Export posts from a specific author (case-insensitive, partial match):
+
+```bash
+# Export posts by "John Doe"
+./seo export --author "John Doe"
+
+# Export posts by "admin" (partial match)
+./seo export --author admin
+
+# Export posts from multiple authors
+./seo export --author "John Doe" "Jane Smith"
+```
+
+### Filter by Author ID
+
+Export posts from specific author IDs:
+
+```bash
+# Export posts by author ID 1
+./seo export --author-id 1
+
+# Export posts from multiple author IDs
+./seo export --author-id 1 2 3
+```
+
+### Combine with Site Filter
+
+Export posts from a specific author on a specific site:
+
+```bash
+# Export John's posts from mistergeek.net only
+./seo export --author "John Doe" --site mistergeek.net
+
+# Export posts by author ID 1 from webscroll.fr
+./seo export --author-id 1 -s webscroll.fr
+```
+
+### Dry Run Mode
+
+Preview what would be exported:
+
+```bash
+./seo export --author "John Doe" --dry-run
+```
+
+## How It Works
+
+1. **Author Name Matching**
+   - Case-insensitive matching
+   - Partial matches work (e.g., "john" matches "John Doe")
+   - Matches against author's display name and slug
+
+2. **Author ID Matching**
+   - Exact match on WordPress user ID
+   - More reliable than name matching
+   - Useful when authors have similar names
+
+3. **Author Information**
+   - The exporter fetches all authors from each site
+   - Author names are included in the exported CSV
+   - Posts are filtered before export
+
+## Export Output
+
+The exported CSV includes author information:
+
+```csv
+site,post_id,status,title,slug,url,author_id,author_name,date_published,...
+mistergeek.net,123,publish,"VPN Guide",vpn-guide,https://...,1,John Doe,2024-01-15,...
+```
+
+### New Column: `author_name`
+
+The export now includes the author's display name in addition to the author ID.
+
+## Examples
+
+### Example 1: Export All Posts by Admin
+
+```bash
+./seo export --author admin
+```
+
+Output: `output/all_posts_YYYY-MM-DD.csv`
+
+### Example 2: Export Specific Author from Specific Site
+
+```bash
+./seo export --author "Marie" --site webscroll.fr
+```
+
+### Example 3: Export Multiple Authors
+
+```bash
+./seo export --author "John" "Marie" "Admin"
+```
+
+### Example 4: Export by Author ID
+
+```bash
+./seo export --author-id 5
+```
+
+### Example 5: Combine Author and Site Filters
+
+```bash
+./seo export --author "John" --site mistergeek.net --verbose
+```
+
+## Finding Author IDs
+
+If you don't know the author ID, you can:
+
+1. **Export all posts and check the CSV:**
+   ```bash
+   ./seo export
+   # Then open the CSV and check the author_id column
+   ```
+
+2. **Use WordPress Admin:**
+   - Go to Users → All Users
+   - Hover over a user name
+   - The URL shows the user ID (e.g., `user_id=5`)
+
+3. **Use WordPress REST API directly:**
+   ```bash
+   curl -u username:password https://yoursite.com/wp-json/wp/v2/users
+   ```
+
+## Tips
+
+1. **Use quotes for names with spaces:**
+   ```bash
+   ./seo export --author "John Doe"  # ✓ Correct
+   ./seo export --author John Doe    # ✗ Wrong (treated as 2 authors)
+   ```
+
+2. **Partial matching is your friend:**
+   ```bash
+   ./seo export --author "john"  # Matches "John Doe", "Johnny", etc.
+   ```
+
+3. **Combine with migration:**
+   ```bash
+   # Export author's posts, then migrate to another site
+   ./seo export --author "John Doe" --site webscroll.fr
+   ./seo migrate output/all_posts_*.csv --destination mistergeek.net
+   ```
+
+4. **Verbose mode for debugging:**
+   ```bash
+   ./seo export --author "John" --verbose
+   ```
+
+## Troubleshooting
+
+### No posts exported
+
+**Possible causes:**
+- Author name doesn't match (try different spelling)
+- Author has no posts
+- Author doesn't exist on that site
+
+**Solutions:**
+- Use `--verbose` to see what's happening
+- Try author ID instead of name
+- Check if author exists on the site
+
+### Author names not showing in CSV
+
+**Possible causes:**
+- WordPress REST API doesn't allow user enumeration
+- Authentication issue
+
+**Solutions:**
+- Check WordPress user permissions
+- Verify credentials in config
+- Author ID will still be present even if name lookup fails
+
+## API Usage
+
+Use author filtering programmatically:
+
+```python
+from seo.app import SEOApp
+
+app = SEOApp()
+
+# Export by author name
+csv_file = app.export(author_filter=["John Doe"])
+
+# Export by author ID
+csv_file = app.export(author_ids=[1, 2])
+
+# Export by author and site
+csv_file = app.export(
+    author_filter=["John"],
+    site_filter="mistergeek.net"
+)
+```
+
+## Related Commands
+
+- `seo migrate` - Migrate exported posts to another site
+- `seo analyze` - Analyze exported posts with AI
+- `seo export --help` - Show all export options
+
+## See Also
+
+- [MIGRATION_GUIDE.md](MIGRATION_GUIDE.md) - Post migration guide
+- [README.md](README.md) - Main documentation