Add assign_ticket and list_agents MCP tools

**Objective:** Add member assignment and discovery to MCP interface (supports agents + humans). **Tools to Add:** 1. **`list_members`** - Get team members - Returns: id, name, role (worker/reviewer/orchestrator/user), active, assigned_count - Filters: role, active (true/false) - Use case: Orchestrator finds available workers 2. **`assign_ticket`** - Assign ticket + optional state transition (2-in-1) - Parameters: - `ticket_id` (required) - `member_id` (optional) → if null, assigns to current_agent - `status` (optional) → if provided, also transitions ticket to this state - Examples: - Worker: `assign_ticket(ticket_id, status: "in_progress")` → self-assign + start - Orchestrator: `assign_ticket(ticket_id, member_id: X, status: "todo")` → assign + queue - Only works on claimable statuses: backlog, todo - Validates member exists and is active - Returns: updated ticket with new assignee and status **Acceptance Criteria:** - `list_members` returns unified agent+user list with role field - `assign_ticket` defaults to current_agent when member_id omitted - `assign_ticket` performs both assignment and transition when status provided - Assignment is logged/audited - Follows existing MCP patterns **Files to Modify:** - `app/controllers/api/v1/mcp_controller.rb`