Add 'stop' command to cancel current work

Allow user to stop in-progress work.

Trigger

User sends: 'stop' or 'cancel'

Behavior

1. Kill the pi-orchestrate subprocess 2. Leave changes uncommitted (don't revert) 3. Report to user with options

Response

'Stopped work on t-123. Changes are uncommitted. [Resume] [Discard] [Review manually]'

Implementation

• Track subprocess ProcessHandle
• Use terminateProcess or interruptProcessGroupOf
• Inline keyboard for next action

Depends on

t-280.2.1 (need orchestrator subprocess infrastructure first)