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

.dockerignore

@@ -44,7 +44,9 @@
 **/[iI][dD]_[rR][sS][aA]
 **/[iI][dD]_[dD][sS][aA]
 **/[iI][dD]_[eE][cC][dD][sS][aA]
+**/[iI][dD]_[eE][cC][dD][sS][aA]_[sS][kK]
 **/[iI][dD]_[eE][dD]25519
+**/[iI][dD]_[eE][dD]25519_[sS][kK]
 
 # Dependencies: restored inside the image, never copied in.
 **/node_modules

.gitignore

@@ -42,4 +42,6 @@ node_modules/
 [iI][dD]_[rR][sS][aA]
 [iI][dD]_[dD][sS][aA]
 [iI][dD]_[eE][cC][dD][sS][aA]
+[iI][dD]_[eE][cC][dD][sS][aA]_[sS][kK]
 [iI][dD]_[eE][dD]25519
+[iI][dD]_[eE][dD]25519_[sS][kK]

TODO.md

@@ -30,6 +30,14 @@ fmt-check, and commit.
   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: `REPO_POLICIES.md` now says how a Go tool a repo needs on the host
+  is pinned (issue 37): installed with `go install` pinned to a commit hash,
+  never tracked as a `go.mod` tool dependency or through a `tools.go` file.
+  golangci-lint is unaffected, since no repo installs it on the host.
+- 2026-10-04: The canonical `.gitignore` and `.dockerignore` now also keep out
+  `id_ecdsa_sk` and `id_ed25519_sk`, the private key files `ssh-keygen` writes
+  for keys backed by a hardware security key (issue 81). Their `.pub` halves
+  stay trackable.
 - 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.

prompts/REPO_POLICIES.md

@@ -495,6 +495,11 @@ style conventions are in separate documents:
 
     Keep it POSIX sh: no arrays, no `[[`, no `grep -P`.
 
+    A Go tool a repo needs on the host is installed with `go install` pinned to
+    a commit hash (`go install <package>@<commit hash>`). It is never tracked as
+    a `go.mod` tool dependency or through a `tools.go` file, either of which
+    pulls the tool's own dependencies into the repo's `go.mod` and `go.sum`.
+
 - When pinning images or packages by hash, add a comment above the reference
   with the version and date (YYYY-MM-DD).