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/services/health.ts

@@ -74,7 +74,7 @@ export async function checkWorkerHealth(opts: {
       fixed: false,
     };
     if (autoFix) {
-      await updateWorker(workspaceDir, groupId, role, { active: false, issueId: null });
+      await updateWorker(workspaceDir, groupId, role, { active: false, issueId: null, startTime: null });
       fix.fixed = true;
     }
     fixes.push(fix);
@@ -95,7 +95,7 @@ export async function checkWorkerHealth(opts: {
       await revertIssueLabel(fix);
       const sessions = { ...worker.sessions };
       if (worker.tier) sessions[worker.tier] = null;
-      await updateWorker(workspaceDir, groupId, role, { active: false, issueId: null, sessions });
+      await updateWorker(workspaceDir, groupId, role, { active: false, issueId: null, startTime: null, sessions });
       fix.fixed = true;
     }
     fixes.push(fix);
@@ -135,7 +135,7 @@ export async function checkWorkerHealth(opts: {
       };
       if (autoFix) {
         await revertIssueLabel(fix);
-        await updateWorker(workspaceDir, groupId, role, { active: false, issueId: null });
+        await updateWorker(workspaceDir, groupId, role, { active: false, issueId: null, startTime: null });
         fix.fixed = true;
       }
       fixes.push(fix);