fix: resolve linter issues (gci import grouping, gofumpt, govet shadow)

- Separate third-party imports from local module imports (gci)
- Fix byte slice literal formatting (gofumpt)
- Rename shadowed err variable to ftErr (govet)
- Remove trailing blank lines in test files

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

pkg/agent/loop_test.go

@@ -906,10 +906,12 @@ func TestResolveMediaRefs_DoesNotMutateOriginal(t *testing.T) {
 	store := media.NewFileMediaStore()
 	dir := t.TempDir()
 	pngPath := filepath.Join(dir, "test.png")
-	pngHeader := []byte{0x89, 0x50, 0x4E, 0x47, 0x0D, 0x0A, 0x1A, 0x0A,
+	pngHeader := []byte{
+		0x89, 0x50, 0x4E, 0x47, 0x0D, 0x0A, 0x1A, 0x0A,
 		0x00, 0x00, 0x00, 0x0D, 0x49, 0x48, 0x44, 0x52,
 		0x00, 0x00, 0x00, 0x01, 0x00, 0x00, 0x00, 0x01, 0x08, 0x02,
-		0x00, 0x00, 0x00, 0x90, 0x77, 0x53, 0xDE}
+		0x00, 0x00, 0x00, 0x90, 0x77, 0x53, 0xDE,
+	}
 	os.WriteFile(pngPath, pngHeader, 0o644)
 	ref, _ := store.Store(pngPath, media.MediaMeta{}, "test")
 
@@ -947,4 +949,3 @@ func TestResolveMediaRefs_UsesMetaContentType(t *testing.T) {
 		t.Fatalf("expected jpeg prefix, got %q", result[0].Media[0][:30])
 	}
 }
-