buzz-acp: channel wedges permanently when a session's agent child dies (-32603 retried into the same dead session until dead-letter)

# Draft: buzz-acp: channel wedges permanently when a session's agent child dies (-32603 retried into the same dead session until dead-letter)

**Component:** buzz-acp (crates/buzz-acp), observed at desktop-v0.5.3
**Severity:** channel outage until process restart, silent message loss

## Summary
If a session's underlying agent child dies out-of-band (in our case: an operator killed claude session processes), the next `session/prompt` returns JSON-RPC `-32603 "Internal error"`. buzz-acp classifies every `AgentError` as "application error — pipe intact", returns the agent to the pool, and crucially leaves `SessionState.sessions[channel_id]` pointing at the dead session:

- `pool.rs` `run_prompt_task` error arm: `if !matches!(e, AcpError::AgentError { .. }) { agent.state.invalidate(&source); }` — the healthy-session exemption also covers dead sessions.
- The requeued batch (`MAX_RETRIES = 10`, exponential backoff) re-enters `run_prompt_task`, hits the session cache (`sessions.get(cid)`), and re-prompts the same dead session every attempt until the batch dead-letters.
- There is no per-session liveness probe and no `session/load`, so nothing ever heals the map short of restarting the whole process.

Real-world impact for us: two channels deaf for ~4 hours, 52 owner events dead-lettered.

## Suggested fix (minimal)
Treat `-32603` as session-invalidating: `matches!(e, AcpError::AgentError { code, .. } if *code == -32603)` → `agent.state.invalidate(&source)`. The already-existing requeue path then misses the session cache and `create_session_and_apply_model` builds a fresh session on the next attempt. False positives are cheap (one extra `session/new`); false negatives wedge the channel. The proactive `max_turns_per_session` rotation already proves the invalidate-and-recreate idiom.

We are running this patch locally and can PR it if you'd take it.