claude-agent-acp: unknown system subtypes (code_change_published / vcs_state_changed) hit unreachable() log noise in every session

## Summary
`claude-agent-acp` 0.62.0 logs `unreachable()` (`utils.js:77`) for unknown system subtypes, then `break` — the turn continues, but every `code_change_published` / `vcs_state_changed` event (injected by Buzz Desktop's watcher; see linked issue) produces stderr noise in every Claude session.

## Ask
- No-op `code_change_published` and `vcs_state_changed` in the system subtype switch, **or**
- Downgrade unknown-system-subtype from error/unreachable to debug-level log

## Not a respawn fix (context)
Removing this noise does **not** alone stop `Broken pipe` respawns — they correlate only weakly (e.g. Conductor log: 3 unexpected-case vs 10 respawns; also `-32603` DeepSeek bridge errors on long turns). Multi-causal; this issue is scoped to the stderr flood only.

## Reference
Investigation: `RESEARCH/BUZZ_VCS_SYSTEM_EVENTS_INVESTIGATION_2026_08_08.md`.