fix(concurrency): release slot on subprocess SIGTERM failure (#40)

Per forensic analysis in #37, the timeout handler at server.mjs:471
called `proc.kill('SIGTERM')` without decrementing the concurrency
counter (`stats.activeRequests`). If the subprocess was stuck in a
syscall (e.g. MCP I/O) and ignored SIGTERM, its slot was not freed
until — and only if — the `proc.on('close')` handler eventually fired
after the 5s SIGKILL escalation successfully reaped the child.
Real-world observation (#37) shows 3 stuck claude subprocesses running
20 minutes to 2h 45m, exhausting the 8-slot pool and returning
`500 concurrency limit reached (8/8)` on every subsequent request.

Fix: attach `cleanup` to `proc.once('exit', ...)`. The 'exit' event
fires before 'close' and runs on every termination path — normal
completion, error, SIGTERM, SIGKILL, crash — even if stdio pipes
stay open. `cleanup()` is already idempotent (guarded by `cleaned`
flag), so this listener is safe alongside the existing 'close' and
'error' paths that also call it.

ALIGNMENT: cli.js has no equivalent subprocess-pool / concurrency-
limit logic — cli.js is a single-user CLI, the process being pooled,
not the pool itself. Per ALIGNMENT.md Rule 2, this fix is proxy-
internal (subprocess lifecycle management) with no cli.js equivalent
to cite; documented here instead. No wire-protocol, HTTP surface, or
response shape change (Rule 3 preserved).

Release kit: bumps version to 3.11.1 and adds CHANGELOG entry so the
auto-release workflow tags v3.11.1 on merge. README version refs are
feature-gate markers for v3.11.0 (models.json SPOT / auto-sync) and
remain historically accurate — no README change needed.

Fixes #37.

Co-authored-by: dtzp555 <dtzp555@gmail.com>

CHANGELOG.md

@@ -1,5 +1,10 @@
 # Changelog
 
+## v3.11.1 — 2026-04-21
+
+### Fixes
+- Concurrency slot leak on subprocess timeout (#37). The request-timeout handler called `proc.kill("SIGTERM")` without decrementing `stats.activeRequests`. A subprocess stuck in a syscall that ignored SIGTERM would hold its slot until (or beyond) the 5s SIGKILL escalation actually reaped it. Slot release is now wired to `proc.once("exit", cleanup)` so every termination path — normal close, error, SIGTERM, SIGKILL — releases the slot exactly once.
+
 ## v3.11.0 — 2026-04-20
 
 ### Features