Bake git commit into the binary and log it at startup

New internal/version package: ldflags-stamped Commit with a
debug.ReadBuildInfo VCS fallback for host builds. Startup log line
carries commit + Go version; --version prints the hash and exits.
Makefile computes GIT_COMMIT (12 chars, -dirty on any local change) and
passes it to docker-build/buildx; Dockerfile injects it via -ldflags and
an org.opencontainers.image.revision label. make build now uses ./cmd —
file-argument builds skip Go's automatic VCS stamp.

Co-Authored-By: Claude <noreply@anthropic.com>

internal/version/version.go

@@ -0,0 +1,47 @@
+// Package version reports which commit the binary was built from. Docker
+// builds stamp it via -ldflags (the build context has no .git, so Go's
+// automatic VCS stamp is absent there); host builds fall back to that
+// automatic stamp.
+package version
+
+import "runtime/debug"
+
+// Commit is set at link time via
+// -ldflags "-X <module>/internal/version.Commit=<hash>".
+var Commit string
+
+// Resolve returns the commit the binary was built from, or "unknown" when
+// neither the ldflags stamp nor build info is available (e.g. go test).
+func Resolve() string {
+	return resolve(Commit, debug.ReadBuildInfo)
+}
+
+func resolve(ldflagsCommit string, readBuildInfo func() (*debug.BuildInfo, bool)) string {
+	if ldflagsCommit != "" {
+		return ldflagsCommit
+	}
+	bi, ok := readBuildInfo()
+	if !ok {
+		return "unknown"
+	}
+	var revision string
+	var modified bool
+	for _, s := range bi.Settings {
+		switch s.Key {
+		case "vcs.revision":
+			revision = s.Value
+		case "vcs.modified":
+			modified = s.Value == "true"
+		}
+	}
+	if revision == "" {
+		return "unknown"
+	}
+	if len(revision) > 12 {
+		revision = revision[:12]
+	}
+	if modified {
+		revision += "-dirty"
+	}
+	return revision
+}