Replace all 6 task tool stubs (TaskCreate/Get/List/Stop/Update/Output)
with real TaskRegistry-backed implementations:
- TaskCreate: creates task in global registry, returns real task_id
- TaskGet: retrieves full task state (status, messages, timestamps)
- TaskList: lists all tasks with metadata
- TaskStop: transitions task to stopped state with validation
- TaskUpdate: appends user messages to task message history
- TaskOutput: returns accumulated task output
Global registry uses OnceLock<TaskRegistry> singleton per process.
All existing tests pass (37 tools, 149 runtime, 102 CLI).
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