fix(cron): wait for spawned subagents before marking cron job complete

[michaelxer]

Problem

Cron jobs that spawn subagents via the spawn tool are marked as completed as soon as the agent's main turn finishes (process_direct returns), while the subagent's asyncio.Task is still running in the background.

From the logs in #4290:

14:00:31  Spawned subagent [77dba3c5]: cron-2pm
14:00:44  Cron: job 'cron-2pm' completed    ← too early
14:02:38  Subagent [77dba3c5] completed successfully

The cron service records the run as "ok" and advances to the next scheduled time before the subagent has produced any output. If the job is one-shot (kind: "at") it may even be deleted before the subagent finishes.

Fixes #4290

Fix

SubagentManager already tracks per-session background tasks in _session_tasks. This PR adds:

1. SubagentManager.await_by_session(session_key) -- waits for all running tasks spawned under that session key to finish (mirrors the existing cancel_by_session).

2. Call in on_cron_job -- after agent.process_direct() returns, the cron handler now calls await agent.subagents.await_by_session(f"cron:{job.id}") before evaluating the response or returning. The cron service therefore holds the job open until every background subagent completes.

This does not change the spawn tool's behaviour for interactive sessions -- only the cron path awaits.

Testing

Four new tests in tests/agent/test_subagent_lifecycle.py (TestAwaitBySession):

Test	Verifies
test_waits_for_running_tasks	Blocks until the subagent task finishes
test_no_tasks_returns_immediately	No-op when no subagents exist for the session
test_already_done_returns_immediately	No-op when subagents finished before the call
test_waits_for_multiple_tasks	Waits for all tasks, not just the first

Full suite: 122 passed (subagent + cron + lifecycle tests).

Limitations

This fix ensures the cron job waits for subagent completion. It does not yet feed the subagent's result back into the agent's cron turn as additional context -- that would require a multi-turn cron execution model and is a separate enhancement.