…ask Group 4) (#8)

* feat: add custom message type filtering to hook system (Phase 3, Task Group 3)

- Add custom_message_type field to HookFilter struct
- Implement extract_custom_message_type() for namespace detection
- Update matches_filter() to check custom types against expected types
- Add unit tests for custom type extraction and filtering (6 new tests)
- Add integration test for E2E custom type routing
- Fix clippy warnings in existing filter tests

Phase 3 Task Group 3 complete. Hook system now supports filtering
on all 12 custom XEP message types (thought, tool_call, tool_result,
todo, code_change, integration, review_comment, work_available,
question, answer, status_update, feature_complete) defined in Phase 3.1.

Test Results:
- 20 unit tests pass (chattermax-server)
- 1 integration test passes
- 15 chattermax-core tests pass
- Zero compiler warnings
- Clippy passes with -D warnings
- Code formatted with cargo fmt

* feat: extract chattermax-client library with public API

- Created Client struct with async connect, send_message, send_custom_message
- Added ClientBuilder for ergonomic configuration
- Created lib.rs exposing public API with re-exported core types
- Updated error handling to use library-specific ClientError
- Made Connection module internal (pub(crate)) with send_raw method
- Prepared foundation for CLI to use library (Phase 3 Task Group 4)

Library features:
- ClientBuilder::new(server, username, password) with port/tls options
- Client::send_message() for plain text
- Client::send_custom_message() for 12 XEP types
- Client::join_room/leave_room/close for room management
- Graceful error handling with thiserror

Chibi integration ready: Library can be used as dependency to send/receive
structured inter-agent messages over XMPP.