F8: persist controller.yaml as 0600 (holds infra secrets)

The Hub config-apply handler wrote controller.yaml 0644; it holds cf_api_token,
cf_tunnel_token and hub api_key in plaintext. New writeConfig0600 helper writes
0600 atomically (tmp+rename, bind-mount fallback) and chmods to enforce 0600 even
when the file pre-existed 0644 (os.WriteFile doesn't chmod existing files).
Test asserts mode 0600 (Linux; skipped on Windows). Setup path already used 0600.

controller/internal/api/config_perms_test.go

@@ -0,0 +1,39 @@
+package api
+
+import (
+	"os"
+	"path/filepath"
+	"runtime"
+	"testing"
+)
+
+// TestWriteConfig0600 asserts F8: controller.yaml is persisted 0600 (it holds infra secrets), even when
+// the target file already existed with looser (0644) permissions. POSIX modes only — skipped on Windows.
+func TestWriteConfig0600(t *testing.T) {
+	if runtime.GOOS == "windows" {
+		t.Skip("POSIX file modes not represented on Windows")
+	}
+	dir := t.TempDir()
+	path := filepath.Join(dir, "controller.yaml")
+
+	// Pre-create with world-readable 0644 to prove the helper tightens an existing file.
+	if err := os.WriteFile(path, []byte("old: true\n"), 0o644); err != nil {
+		t.Fatal(err)
+	}
+
+	if err := writeConfig0600(path, []byte("hub:\n  api_key: redacted\n")); err != nil {
+		t.Fatalf("writeConfig0600: %v", err)
+	}
+
+	fi, err := os.Stat(path)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if mode := fi.Mode().Perm(); mode != 0o600 {
+		t.Fatalf("config mode = %o, want 0600", mode)
+	}
+	// No leftover temp file.
+	if _, err := os.Stat(path + ".tmp"); !os.IsNotExist(err) {
+		t.Fatalf("temp file not cleaned up")
+	}
+}