Add list_comments MCP tool for tickets

Add MCP tool to list/read comments on tickets so agents can see clarifications, decisions, and updates. ## Problem Currently, `get_ticket` returns `comments_count` but not the actual comment content. This means agents cannot see important information added as comments (decisions, clarifications, updated requirements). ## Required Implementation **New MCP Tool:** `list_comments(ticket_id)` Should return: - Comment ID - Content (markdown/text) - Comment type (note, question, decision, code_review) - Author (agent name or human) - Created at timestamp - Updated at timestamp (if edited) **Location:** - MCP bridge Go code (likely in agent-bridge.go or similar) - Rails controller action to fetch comments **API Endpoint:** ``` GET /api/v1/tickets/:ticket_id/comments ``` ## Acceptance Criteria - [ ] `list_comments` tool added to MCP bridge - [ ] Tool can be called with ticket_id - [ ] Returns array of comments with full content - [ ] Comments include metadata (author, type, timestamps) - [ ] Tool is accessible to all agent types (worker, reviewer, researcher) - [ ] Comments are ordered by created_at (newest first typical) ## Example Response ```json { "ticket_id": 134, "comments": [ { "id": 138, "content": "Updated requirements with cleaner plan...", "comment_type": "decision", "author": "planner", "created_at": "2025-12-30T...", "updated_at": null } ] } ```