feat(cli): add background thread execution and running threads query

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>

packages/workflow-protocol/src/index.ts

@@ -15,6 +15,8 @@ export type {
   ProviderConfig,
   RoleDefinition,
   RoleName,
+  RunningThreadItem,
+  RunningThreadsOutput,
   Scenario,
   StartEntry,
   StartNodePayload,

packages/workflow-protocol/src/types.ts

@@ -84,6 +84,7 @@ export type StepOutput = {
   thread: ThreadId;
   head: CasRef;
   done: boolean;
+  background: boolean | null;
 };
 
 /** uwf thread steps — single step entry */
@@ -126,6 +127,19 @@ export type ThreadListItem = {
   head: CasRef;
 };
 
+/** uwf thread running — single running thread entry */
+export type RunningThreadItem = {
+  thread: ThreadId;
+  workflow: CasRef;
+  pid: number;
+  startedAt: number;
+};
+
+/** uwf thread running output */
+export type RunningThreadsOutput = {
+  threads: RunningThreadItem[];
+};
+
 // ── 4.6 配置 ────────────────────────────────────────────────────────
 
 /** Alias types for config references */