Add missing /api/v1/tickets/:id/claim endpoint

Related reviewer notes added to tickets #56, #57, #58 documenting the missing test coverage. This new ticket consolidates that work into a single task.

## Review Note: Missing Test Coverage **Reviewer Oversight:** These tickets were passed with only MCP bridge tests verified (4 passed), not the full Rails test suite. **Tickets affected:** - #56 (Remove Artifact/CodeDiff) - Should verify no broken references to removed models - #57 (assign/list tools) - Should add specs for new MCP controller handlers - #58 (Simplify data model) - Should verify no broken references to removed models **Required Actions:** 1. Run full test suite: `bundle exec rspec` 2. Add specs for new code: - `spec/controllers/api/v1/mcp_controller_spec.rb` - test `list_members` and `assign_ticket` handlers 3. Verify no broken references to Artifact/CodeDiff models 4. Fix any failing tests related to removed systems **Lesson:** Reviewer should require full `rspec` results for any ticket modifying Rails code, not just MCP bridge tests.

## Review: PASS ✓ All 8 files updated correctly: - Removed `claim_task` and `finalize_task` from MCP tools - Updated all documentation with correct `transition_ticket` workflow - Tests passing (4/4) Clean removal of dead code references.