docs: add comprehensive keyword filtering specification

- Update CLAUDE.md with keyword filtering in Next Steps section
- Add detailed TODO.md with feature specification, use cases, and implementation notes
- Document subjectKeywords, senderKeywords, and recipientKeywords functionality
- Include JSON configuration examples and priority assessment

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

TODO.md

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+# mail2couch TODO and Feature Requests
+
+## Planned Features
+
+### Keyword Filtering for Messages
+
+Add support for filtering messages by keywords in various message fields. This would extend the current `messageFilter` configuration.
+
+**Proposed Configuration Extension:**
+
+```json
+{
+  "messageFilter": {
+    "since": "2024-01-01",
+    "subjectKeywords": ["urgent", "important", "meeting"],
+    "senderKeywords": ["@company.com", "notifications"],
+    "recipientKeywords": ["team@company.com", "all@"]
+  }
+}
+```
+
+**Implementation Details:**
+
+- `subjectKeywords`: Array of keywords to match in email subject lines
+- `senderKeywords`: Array of keywords to match in sender email addresses or names  
+- `recipientKeywords`: Array of keywords to match in recipient (To/CC/BCC) addresses or names
+- Keywords should support both inclusive (must contain) and exclusive (must not contain) patterns
+- Case-insensitive matching by default
+- Support for simple wildcards or regex patterns
+
+**Use Cases:**
+
+1. **Corporate Email Filtering**: Only backup emails from specific domains or containing work-related keywords
+2. **Project-based Archiving**: Filter emails related to specific projects or clients
+3. **Notification Management**: Exclude or include automated notifications based on sender patterns
+4. **Security**: Filter out potential spam/phishing by excluding certain keywords or senders
+
+**Implementation Priority:** Medium - useful for reducing storage requirements and focusing on relevant emails.
+
+## Other Planned Improvements
+
+1. **Real IMAP Message Parsing**: Replace placeholder data with actual message content
+2. **Message Body Extraction**: Support for HTML/plain text and multipart messages  
+3. **Attachment Handling**: Optional support for email attachments
+4. **Batch Operations**: Improve CouchDB insertion performance
+5. **Error Recovery**: Retry logic and partial sync recovery
+6. **Testing**: Comprehensive unit test coverage