Background daemon: fresh-spawned (non-spare) --bg workers crash "exit N before init" 100% of the time

**Version:** 2.1.212 (also observed on 2.1.14x-2.1.15x daemons; macOS 14, Darwin 23.6.0)

**Summary**

The background daemon keeps one pre-warmed spare worker. Jobs that claim the spare (`bg claimed-spare ` in daemon.log) succeed. Jobs dispatched through the fresh-spawn path (`bg spawned `) crash roughly 13 seconds later with `bg settled (crashed): exit N before init`, before session init, every single time. In my full daemon.log history: 394 spare-claimed jobs, almost all successful; 16 fresh-spawned jobs, 16 crashed. Zero fresh-spawn successes on record.

**Reproduction conditions**

A job lands on the fresh-spawn path in two cases, both reproducible on my machine:

1. Two `claude --bg` dispatches arrive in the same second. The first claims the spare and succeeds; the second fresh-spawns and crashes. Observed daily when two scheduled jobs fire at the same minute; the crashing job alternates depending on which dispatch wins the race.
2. Cold daemon start (no spare exists yet). The dispatching job fresh-spawns and crashes.

**Sample log window (2026-08-27, UTC)**

```
14:00:05.726 [bg] bg claimed-spare d8fe41c4 (shell) <- succeeded (ran 7m, done)
14:00:05.745 [bg] bg spawned f6eba5ef (shell) <- fresh spawn
14:00:05.760 [bg] bg spare spawned host pid=16857
14:00:18.794 [bg] bg settled f6eba5ef (crashed): exit 1 before init
```

Auth was healthy in the same window (`auth: scheduling proactive refresh in 28560s`, no failure lines), so this is not the headless OAuth-refresh issue. Immediately after past occurrences, direct `claude -p` and `claude --bg` probes with the identical flag set completed clean, so flags, model, prompt, and credentials are not the cause.

**Impact**

Any consumer that schedules more than one background job at the same minute, or dispatches into a cold daemon, deterministically loses a job. The worker's stderr is not observable from the dispatching side (the launcher's pipes are destroyed at the `backgrounded` banner), so the only diagnostic available is the `exit N before init` detail string.

**Ask**

Either fix the fresh-spawn worker path, or have the daemon queue the second dispatch until the replenishment spare is ready, or surface the worker's early stderr in `state.json` so consumers can see why init failed.

Start by reproducing the cold-daemon and two-dispatch cases with `claude --bg`, then inspect the daemon log entries for `bg spawned`, `bg claimed-spare`, and `exit N before init`. Trace the fresh-spawn path and the `state.json` reporting path; done means fresh-spawned workers initialize reliably or their early failure is made observable, with queued dispatches handled if that is the chosen behavior.

索引モデルが issue の本文から書いたものです。