Terminate MCP child process trees on codex session close

[ojowwalker77]

Closes #21 (partial — core teardown; see Deferred).

Problem

On POSIX, closing/cancelling/replacing a codex session sent a bare SIGTERM to only the codex app-server process — no tree/group kill, no forced escalation. Any MCP server the agent CLI spawned as a grandchild could orphan and accumulate across sessions, degrading low-RAM (8 GB) Macs. killChildTree was Windows-only for tree teardown; POSIX fell through to child.kill().

Fix

• childProcessTreeTerminator.ts (new): captures a child's descendant tree before signaling, sends graceful SIGTERM to the process group + tree + handle, then force-SIGKILLs survivors after a bounded grace window. Command-verifies captured PIDs before SIGKILL to avoid killing a reused PID, and skips the root's pid/group once it has exited. Reuses the tested terminal/processTreeKiller primitive instead of duplicating it; emits structured lifecycle logs (threadId, kind, pid/pgid, descendants, signal, graceMs).
• spawnCodexAppServer: now spawns detached: true on POSIX so the app-server leads its own process group — teardown can signal codex + its MCP children as a unit in one call. Never unref'd; lifecycle stays owned by the manager. Windows keeps taskkill /T /F.
• Codex teardown (stopSession / stopDiscoverySession) routes through the shared terminator with session-scoped logs.

Graceful app shutdown is already covered: NodeRuntime.runMain runs the stopAll finalizer on SIGINT/SIGTERM, which now performs a real tree kill.

Behavioral note

Codex app-server is now spawned detached on POSIX (its own process group). Functionally transparent for a server (no controlling TTY), called out for review visibility.

Tests

• New childProcessTreeTerminator.test.ts: graceful group/tree/handle signaling, SIGTERM→SIGKILL escalation, post-exit captured-descendant-only reaping (PID-reuse safety), escalation cancellation, no-pid skip, Windows taskkill path, and a real detached child-tree integration test (spawns sh + sleep grandchild, verifies the whole tree exits).
• Existing codexAppServerManager.test.ts (71) still pass.
• oxfmt, oxlint (0 warnings in new files), turbo run typecheck (8/8) green.

Deferred (need cross-restart infrastructure)

Out of scope for this PR; the teardown fix eliminates the leak on every normal path (the reaper only matters after a hard crash/SIGKILL of the server):

• Startup orphan reaping (requires a persisted PID registry).
• Reference-counted shared/global MCP services.
• macOS packaged-app verification + 5× open/close leak assertion.