feat(taskctl): show last agent message time and message count in taskctl status

[randomm]

Task Description

When a developer or adversarial session is assigned, the PM has no visibility into whether the agent is actively working. Showing last message time and message count would let PM detect stuck agents without watching the UI.

The session system already stores this data (messages queryable via session ID).

Implementation

• In taskctl status, for tasks with an assignee, query the session's message count and last message timestamp
• Display as: Agent: 12 messages, last active 2m ago
• Gracefully handle sessions that have ended: Agent: session ended

Quality Gates (Non-Negotiable)

• TDD: Write tests before implementation
• Linting passes
• Local verification: all tests pass

Acceptance Criteria

• taskctl status shows message count and last message time for assigned sessions
• Gracefully handles ended sessions
• No performance regression (one session query per assigned task)