fix(mcp): exit on uncaught exception instead of orphaning/spinning at 100% CPU (#855)

The process-wide uncaughtException handler logged the error and kept running. For
the detached `serve --mcp` daemon that turned any escaped fault into an
unrecoverable orphan: nothing respawns it, and when logging the raw Error hit a
V8 source-position loop while lazily formatting `.stack`, the main thread wedged
at 100% CPU so even the PPID watchdog / idle-timer could no longer fire. Same
failure mode as #799, which only fixed the stdin-'error' trigger.

Restore Node's default fatal semantics: render a bounded, hang-proof line (name +
message only — never read `.stack`) then exit non-zero, so a fresh daemon starts
on the next connection. Extracted to src/bin/fatal-handler.ts with injectable
seams; unit-tested incl. the never-touch-stack invariant.

Closes #850.

CHANGELOG.md

@@ -9,6 +9,10 @@ and adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
 ## [Unreleased]
 
+### Fixes
+
+- The CodeGraph MCP server no longer risks getting stuck at 100% CPU after an unexpected internal error. Previously such an error was logged but the process was left running in a broken state, where it could spin a CPU core indefinitely and had to be killed by hand. The server now logs the error and exits cleanly, so a fresh one starts on the next request. Thanks @songhlc. (#850)
+
 
 ## [1.0.0] - 2026-06-12