Install pinned goimports in script/bootstrap (closes #184)

script/fmt runs gofmt, goimports, and npx prettier, but bootstrap
installed only git, make, go, and golangci-lint, so `make fmt` failed
with `goimports: not found` on a fresh machine. bootstrap's contract is
to install all dependencies idempotently.

Add ensure_goimports: it skips when goimports is already on PATH,
otherwise `go install`s golang.org/x/tools/cmd/goimports at a pinned
exact version (v0.49.0; integrity via the Go module checksum database)
and places the binary in /usr/local/bin so it is on PATH regardless of
shell config, mirroring the golangci-lint release install. v0.49.0
requires Go 1.25, matching go.mod; v0.50.0 would force a Go 1.26
toolchain download.

Model: opus-4-8

TODO.md

@@ -22,6 +22,9 @@ main cannot regress.
 
 - 2026-09-22: Added `.prettierignore` so `make fmt` no longer rewrites
   the vendored `static/js/alpine.min.js` bundle (#185).
+- 2026-09-22: `script/bootstrap` now installs a pinned `goimports`
+  (`golang.org/x/tools` v0.49.0) into `/usr/local/bin`, so `make fmt`
+  succeeds on a fresh machine after `make bootstrap` (#184).
 - 2026-09-09: Fixed four deployability blockers found by QA: CSRF origin
   check over plain HTTP (`UPAAS_PLAINTEXT_HTTP`, #189), pulling the git
   image when absent (#190), the env-var editor CSRF token lookup (#191),