fix: clear startTime when deactivating workers to prevent stale timestamps

Problem:
When workers were deactivated (task completed or fixed by health checks),
the startTime field was not being cleared. This caused:
- Inactive workers to retain stale timestamps
- Misleading duration data in projects.json
- Potential confusion in health checks and status displays

Example from projects.json:
{
  "qa": {
    "active": false,
    "issueId": null,
    "startTime": "2026-02-10T08:51:50.725Z",  // Stale!
    "tier": "qa"
  }
}

Root Cause:
The deactivateWorker() function only set active: false and issueId: null,
but did not clear startTime. Similarly, health check auto-fixes that
deactivated workers also failed to clear startTime.

Solution:
Always set startTime: null when deactivating a worker to ensure clean state.

Changes:
1. lib/projects.ts:
   - deactivateWorker() now sets startTime: null
   - Updated function comment to document this behavior

2. lib/services/health.ts:
   - All three auto-fix paths that deactivate workers now clear startTime:
     * active_no_session fix (line 77)
     * zombie_session fix (line 98)
     * stale_worker fix (line 138)

Impact:
- Inactive workers now have clean state (startTime: null)
- Duration calculations only apply to active workers
- Health checks work with accurate data
- No stale timestamps persisting across task completions
- Complements fix from #108 (which ensures startTime is set on activation)

Together with #108:
- #108: Always SET startTime when activating worker
- #113: Always CLEAR startTime when deactivating worker
- Result: startTime accurately reflects current task duration

Addresses issue #113

lib/projects.ts

@@ -230,6 +230,7 @@ export async function activateWorker(
 /**
  * Mark a worker as inactive after task completion.
  * Preserves sessions map and tier for reuse via updateWorker's spread.
+ * Clears startTime to prevent stale timestamps on inactive workers.
  */
 export async function deactivateWorker(
   workspaceDir: string,
@@ -239,6 +240,7 @@ export async function deactivateWorker(
   return updateWorker(workspaceDir, groupId, role, {
     active: false,
     issueId: null,
+    startTime: null,
   });
 }