Let fx own signals and the exit code in the server example (closes #86)

The lifecycle example computed an exit code that never reached os.Exit,
installed its own SIGINT/SIGTERM handler beside the one fx's Run()
installs, and exited from a goroutine when Sentry failed to start, so
no stop hook ran. Now only fx handles those signals; a listen error
asks fx to shut down with exit code 1 through fx.Shutdowner; Sentry's
error is returned from the server's start hook; and the server's stop
hook shuts the HTTP server down within 5 seconds and fails when
requests are still running. A new paragraph says who owns signals and
the exit code. SIGPIPE is still ignored, now in main.

Model: opus-5-5

TODO.md

@@ -21,6 +21,15 @@ fmt-check, and commit.
 
 # Completed Steps
 
+- 2026-10-04: Fixed the server lifecycle example in
+  `prompts/GO_HTTP_SERVER_CONVENTIONS.md` (issue 86). Only fx handles SIGINT and
+  SIGTERM, and `Run()` in `main` exits with the shutdown's exit code. A listen
+  error asks fx to shut down with exit code 1 through `fx.Shutdowner`; a Sentry
+  start failure is returned from the server's start hook instead of calling
+  `os.Exit` from a goroutine, so the stop hooks of what had started still run.
+  The server's stop hook shuts the HTTP server down within 5 seconds and fails
+  when requests are still running. A new paragraph says who owns signals and the
+  exit code.
 - 2026-10-04: `package.json` now has `"license": "MIT"`, matching `LICENSE`, so
   yarn no longer prints "No license field" when `script/bootstrap` runs it
   inside the Docker phases (issue 76). That was the only yarn warning there.