dimos stop CLI command (by PID or most recent)

[dimos-bot]

Add dimos stop command to stop running dimos instances:

• dimos stop - stops most recent dimos instance
• dimos stop --pid <PID> - stops specific process
• dimos stop --task <TASK_ID> - stops by task ID (if applicable)

Success criteria:

• Clean shutdown of dimos daemon
• Works with both PID and implicit "most recent" mode
• Returns success/failure status

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Synced from DIM-664