This commit implements issue #456, adding two related capabilities to the uwf CLI:
1. **Background execution mode** for `uwf thread step` (via `--background` flag)
- Spawns agent execution in a detached background process
- Returns immediately with thread ID and background status
- Maintains marker files to track running processes
- Supports `--count` option to run multiple steps in background
- Prevents concurrent execution of the same thread
2. **Running threads query** command (`uwf thread running`)
- Lists all threads currently executing in background
- Returns thread ID, workflow, current role, PID, and start time
- Automatically filters out stale markers (dead processes)
- Empty list when no threads are running
**Key changes:**
- **workflow-protocol**: Added `RunningThreadItem`, `RunningThreadsOutput` types
Updated `StepOutput` to include `background: boolean | null` field
- **cli-workflow/background**: New module for process management
- Marker file creation/deletion (atomic operations)
- PID liveness checking
- Stale marker cleanup
- Running threads query
- **cli-workflow/commands/thread**:
- Updated `cmdThreadStep` to support `--background` and `--_background-worker` flags
- Added `cmdThreadStepBackground` for spawning detached processes
- Added `cmdThreadRunning` to list running threads
- Updated `cmdThreadKill` to terminate background processes
- **cli-workflow/cli**: Added CLI routing for new commands and flags
**Integration:**
- `uwf thread kill` now terminates background processes before archiving
- Foreground execution checks for existing background process and fails if found
- Background worker creates/cleans up marker files automatically
- Marker files stored in `~/.uncaged/workflow/running/*.json`
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
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