feat(secrets): file-backed SecretStore for Stage-0

Implements ports.SecretStore over a 0600 JSON file as a stand-in for op/ESO so
the demo runs without live op. Put persists atomically (temp + rename) and
merges; Get returns ErrNotFound for unknown refs so a missing token fails loud.
Behind the port, so swapping to op/ESO later is wiring, not code (ADR-002).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

internal/adapters/secrets/file_test.go

@@ -0,0 +1,72 @@
+package secrets_test
+
+import (
+	"context"
+	"errors"
+	"os"
+	"path/filepath"
+	"testing"
+
+	"gitea.d-ma.be/mathias/tapir/internal/adapters/secrets"
+)
+
+func TestPutThenGet(t *testing.T) {
+	path := filepath.Join(t.TempDir(), "nested", "secrets.json")
+	s := secrets.NewFileStore(path)
+
+	if err := s.Put("youtube/refresh_token", "rt-123"); err != nil {
+		t.Fatalf("Put: %v", err)
+	}
+	got, err := s.Get(context.Background(), "youtube/refresh_token")
+	if err != nil {
+		t.Fatalf("Get: %v", err)
+	}
+	if got != "rt-123" {
+		t.Errorf("Get = %q, want %q", got, "rt-123")
+	}
+}
+
+func TestGetUnknownRef(t *testing.T) {
+	s := secrets.NewFileStore(filepath.Join(t.TempDir(), "secrets.json"))
+	_, err := s.Get(context.Background(), "missing")
+	if !errors.Is(err, secrets.ErrNotFound) {
+		t.Errorf("err = %v, want ErrNotFound", err)
+	}
+}
+
+func TestPutIsOwnerOnly(t *testing.T) {
+	path := filepath.Join(t.TempDir(), "secrets.json")
+	s := secrets.NewFileStore(path)
+	if err := s.Put("k", "v"); err != nil {
+		t.Fatalf("Put: %v", err)
+	}
+	info, err := os.Stat(path)
+	if err != nil {
+		t.Fatalf("Stat: %v", err)
+	}
+	if perm := info.Mode().Perm(); perm != 0o600 {
+		t.Errorf("file perm = %o, want 0600 (token must not be world-readable)", perm)
+	}
+}
+
+func TestPutMergesEntries(t *testing.T) {
+	path := filepath.Join(t.TempDir(), "secrets.json")
+	s := secrets.NewFileStore(path)
+	if err := s.Put("a", "1"); err != nil {
+		t.Fatalf("Put a: %v", err)
+	}
+	if err := s.Put("b", "2"); err != nil {
+		t.Fatalf("Put b: %v", err)
+	}
+	// Re-open from disk to prove persistence, not in-memory state.
+	s2 := secrets.NewFileStore(path)
+	for k, want := range map[string]string{"a": "1", "b": "2"} {
+		got, err := s2.Get(context.Background(), k)
+		if err != nil {
+			t.Fatalf("Get %q: %v", k, err)
+		}
+		if got != want {
+			t.Errorf("Get %q = %q, want %q", k, got, want)
+		}
+	}
+}