fix: stop losing a fast-exiting direct agent's output and close event (#5791)

[atomantic]

Summary

A CLI agent spawned directly (no runner) could vanish into a permanently "running" state when it died instantly — a bad flag, an immediate auth refusal.

In spawnDirectly (server/services/agentCliSpawning.js) the child's error listener was already attached synchronously after spawn(), but stdout, stderr and close were registered far below, past await updateAgent(agentId, { pid }) — real state-file I/O, so it yields for a macrotask, not just a microtask. A child that ran its whole life cycle inside that window had its close event land on nothing: the run record, the execution lane and the activeAgents entry all stayed non-terminal until the orphan reaper eventually noticed. Any stdout/stderr the child wrote before exiting — exactly the output that would have explained the failure — was dropped too.

Every child listener is now attached in the same tick as spawn(), into small forwarding shims that buffer into bindings declared just above. The real handler bodies (which close over a lot of state the async setup builds) are assigned in place as before, then the buffered events are replayed:

• output first, so the transcript is complete — and enqueued onto transcriptWriteTail — before a buffered terminal event finalizes the run;
• exactly one terminal event, since a failed spawn emits error and then close and the error path already finalizes;
• one ordered queue for both streams, so interleaved stdout/stderr chunks keep their emission order.

Scoped to the direct-spawn path. The runner path (server/cos-runner/index.js) attaches its listeners synchronously already and is unaffected.

server/services/agentManagement.test.js's close-handler source contract now anchors on handleClose instead of the claudeProcess.on('close') registration, which is a shim now.

Test plan

• New regression test in server/services/agentCliSpawning.test.js: the fake child emits a stdout chunk and then close from inside the updateAgent(pid) yield, and the test asserts finalizeAgent ran with the right exit code and that the pre-exit output survived in outputBuffer. Verified it fails on pre-fix code (finalizeAgent: 0 calls) and passes after.
• cd server && npm test — 1888 files / 38070 tests passing.

Closes #5791

https://claude.ai/code/session_01Uz59AsJawa1Djqm2T9Fb8t