[TE-5578] Fall back to BUILDKITE_BRANCH when git branch --show-current is empty

In CI, the Buildkite agent checks out a specific commit SHA, leaving
the repo in detached HEAD state where git branch --show-current
returns empty. Fall back to the BUILDKITE_BRANCH env var which the
agent always sets.

Split the DetachedHead test into two cases: one where BUILDKITE_BRANCH
is set (verifies fallback populates the branch field) and one where
it's also empty (verifies branch is absent).

Author: gchan

internal/git/auto_metadata.go

@@ -157,15 +157,24 @@ func mergeNonEmpty(dst, src map[string]string) {
 
 // collectContextFields adds branch, base_branch, and Buildkite env var fields.
 func collectContextFields(ctx context.Context, runner GitRunner, baseBranch string, metadata map[string]string) {
-	// branch: current branch name (empty on detached HEAD, omitted)
+	// branch: current branch name, falling back to BUILDKITE_BRANCH
+	// for detached HEAD (common in CI where the agent checks out a commit SHA)
 	if out, err := runner.Output(ctx, "branch", "--show-current"); err == nil {
-		branch := strings.TrimSpace(out)
-		if branch != "" {
+		if branch := strings.TrimSpace(out); branch != "" {
 			metadata["branch"] = branch
+			debug.Printf("branch resolved via git branch --show-current: %s", branch)
 		}
 	} else {
 		debug.Printf("Warning: git branch --show-current failed: %v", err)
 	}
+	if _, ok := metadata["branch"]; !ok {
+		if branch := os.Getenv("BUILDKITE_BRANCH"); branch != "" {
+			metadata["branch"] = branch
+			debug.Printf("branch resolved via BUILDKITE_BRANCH env var: %s", branch)
+		} else {
+			debug.Printf("branch could not be determined (detached HEAD, no BUILDKITE_BRANCH)")
+		}
+	}
 
 	// base_branch: the resolved base ref (not a git command)
 	if baseBranch != "" {

internal/git/auto_metadata_test.go

@@ -301,9 +301,10 @@ func TestCollectPlanMetadata_LogFails(t *testing.T) {
 	}
 }
 
-func TestCollectPlanMetadata_DetachedHead(t *testing.T) {
+func TestCollectPlanMetadata_DetachedHeadFallsToBuildkiteBranch(t *testing.T) {
 	t.Setenv("BUILDKITE_PIPELINE_SLUG", "")
 	t.Setenv("BUILDKITE_BUILD_ID", "")
+	t.Setenv("BUILDKITE_BRANCH", "feature/from-env")
 
 	gitLogOutput := buildRecord("abc123", "def456", "Alice", "alice@example.com", "2026-03-15T10:00:00+00:00", "Alice", "alice@example.com", "2026-03-15T10:00:00+00:00", "Commit msg")
 
@@ -316,6 +317,40 @@ func TestCollectPlanMetadata_DetachedHead(t *testing.T) {
 
 	got := CollectPlanMetadata(context.Background(), runner, "")
 
+	want := map[string]string{
+		"commit_sha":      "abc123",
+		"parent_shas":     "def456",
+		"author_name":     "Alice",
+		"author_email":    "alice@example.com",
+		"author_date":     "2026-03-15T10:00:00+00:00",
+		"committer_name":  "Alice",
+		"committer_email": "alice@example.com",
+		"committer_date":  "2026-03-15T10:00:00+00:00",
+		"message":         "Commit msg",
+		"branch":          "feature/from-env",
+	}
+
+	if diff := cmp.Diff(want, got); diff != "" {
+		t.Errorf("CollectPlanMetadata mismatch (-want +got):\n%s", diff)
+	}
+}
+
+func TestCollectPlanMetadata_DetachedHeadNoBuildkiteBranch(t *testing.T) {
+	t.Setenv("BUILDKITE_PIPELINE_SLUG", "")
+	t.Setenv("BUILDKITE_BUILD_ID", "")
+	t.Setenv("BUILDKITE_BRANCH", "")
+
+	gitLogOutput := buildRecord("abc123", "def456", "Alice", "alice@example.com", "2026-03-15T10:00:00+00:00", "Alice", "alice@example.com", "2026-03-15T10:00:00+00:00", "Commit msg")
+
+	runner := &FakeGitRunner{
+		Responses: map[string]string{
+			fmt.Sprintf("log -1 --format=%s", MetadataFormat): gitLogOutput,
+			"branch --show-current":                           "\n",
+		},
+	}
+
+	got := CollectPlanMetadata(context.Background(), runner, "")
+
 	want := map[string]string{
 		"commit_sha":      "abc123",
 		"parent_shas":     "def456",