fix(agent): Eliminate race condition in completion handling

PROBLEM:
- Agents completing with questions were incorrectly marked as "crashed"
- Race condition: polling handler AND crash handler both called handleCompletion()
- Caused database corruption and lost pending questions

SOLUTION:
- Add completion mutex in OutputHandler to prevent concurrent processing
- Remove duplicate completion call from crash handler
- Only one handler executes completion logic per agent

TESTING:
- Added mutex-completion.test.ts with 4 test cases
- Verified mutex prevents concurrent access
- Verified lock cleanup on exceptions
- Verified different agents can process concurrently

FIXES: residential-cuckoo and 12+ other agents stuck in crashed state

src/process/manager.test.ts

@@ -389,6 +389,7 @@ describe('ProcessManager', () => {
           payload: {
             processId: 'proc-1',
             pid: 12345,
+            exitCode: 1,
             signal: 'SIGTERM',
           },
         })

src/process/manager.ts

@@ -124,6 +124,7 @@ export class ProcessManager {
             payload: {
               processId: id,
               pid,
+              exitCode: code,
               signal,
             },
           };