Port 7 missing tool definitions from upstream parity audit:
- AskUserQuestionTool: ask user a question with optional choices
- TaskCreate: create background sub-agent task
- TaskGet: get task status by ID
- TaskList: list all background tasks
- TaskStop: stop a running task
- TaskUpdate: send message to a running task
- TaskOutput: retrieve task output
All tools have full ToolSpec schemas registered in mvp_tool_specs()
and stub execute functions wired into execute_tool(). Stubs return
structured JSON responses; real sub-agent runtime integration is the
next step.
Closes parity gap: 21 -> 28 tools (upstream has 40).
fmt/clippy/tests all green.
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