feat(datastore): add UpdateCheckState persistence

First piece of #591 (opt-in periodic update check). A small persisted
state (last_checked_at, last_seen_version) under update-check.json,
mirroring Settings' Get/Save shape — separate from Settings itself since
this is runtime state, not operator-editable config.

Not wired to anything yet.

Refs #591

data/.gitignore

@@ -8,3 +8,4 @@ parity_mismatches/
 stats/
 patterns.json
 settings.json
+update-check.json

pkg/service/datastore/datastore.go

@@ -2720,6 +2720,62 @@ func (ds *DataStore) SaveSettings(settings Settings) error {
 	return ds.atomicWriteFile(path, data)
 }
 
+// UpdateCheckState is the small persisted state for the opt-in periodic
+// update check (#591, _/i591/design-update-check.md): when it last ran and
+// what it last saw, so a restart doesn't lose the "already logged this
+// version" and "don't hammer GitHub on every startup" context. Separate
+// from Settings, which is operator-editable config, not runtime state.
+type UpdateCheckState struct {
+	LastCheckedAt   string `json:"last_checked_at,omitempty"`
+	LastSeenVersion string `json:"last_seen_version,omitempty"`
+}
+
+// GetUpdateCheckState retrieves the persisted update-check state. Same
+// missing-file-is-not-an-error shape as GetSettings — a fresh install (or
+// one that has never had the check enabled) has no file yet.
+func (ds *DataStore) GetUpdateCheckState() (UpdateCheckState, error) {
+	if ds == nil || ds.DataDir == "" {
+		return UpdateCheckState{}, nil
+	}
+
+	path := filepath.Join(ds.DataDir, "update-check.json")
+	if !ds.rootExists(path) {
+		return UpdateCheckState{}, nil
+	}
+
+	data, err := ds.rootReadFile(path)
+	if err != nil {
+		return UpdateCheckState{}, err
+	}
+
+	var state UpdateCheckState
+	if err := json.Unmarshal(data, &state); err != nil {
+		return UpdateCheckState{}, err
+	}
+
+	return state, nil
+}
+
+// SaveUpdateCheckState persists the update-check state.
+func (ds *DataStore) SaveUpdateCheckState(state UpdateCheckState) error {
+	if ds == nil || ds.DataDir == "" {
+		return nil
+	}
+
+	if err := ds.rootMkdirAll(ds.DataDir, 0755); err != nil {
+		return fmt.Errorf("failed to create data directory: %w", err)
+	}
+
+	path := filepath.Join(ds.DataDir, "update-check.json")
+
+	data, err := json.MarshalIndent(state, "", "  ")
+	if err != nil {
+		return err
+	}
+
+	return ds.atomicWriteFile(path, data)
+}
+
 // SaveUsageStats saves usage statistics to the datastore.
 func (ds *DataStore) SaveUsageStats(stats models.UsageStats) error {
 	dir := filepath.Join(ds.DataDir, "stats", "usage")

pkg/service/datastore/datastore_test.go

@@ -458,6 +458,57 @@ func TestSettingsPersistence(t *testing.T) {
 	}
 }
 
+// TestUpdateCheckState_MissingFileReturnsZeroValue verifies a fresh install
+// (or one where the update check has never run) gets a zero-value state,
+// not an error — same shape as GetSettings on a missing settings.json.
+func TestUpdateCheckState_MissingFileReturnsZeroValue(t *testing.T) {
+	tempDir, err := os.MkdirTemp("", "update-check-missing-test-*")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer os.RemoveAll(tempDir)
+
+	ds := NewDataStore(tempDir)
+
+	state, err := ds.GetUpdateCheckState()
+	if err != nil {
+		t.Fatalf("GetUpdateCheckState on a fresh install should not error, got: %v", err)
+	}
+	if state != (UpdateCheckState{}) {
+		t.Errorf("Expected zero-value state, got %+v", state)
+	}
+}
+
+// TestUpdateCheckState_Persistence is the roundtrip test, mirroring
+// TestSettingsPersistence.
+func TestUpdateCheckState_Persistence(t *testing.T) {
+	tempDir, err := os.MkdirTemp("", "update-check-persist-test-*")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer os.RemoveAll(tempDir)
+
+	ds := NewDataStore(tempDir)
+
+	state := UpdateCheckState{
+		LastCheckedAt:   "2026-08-09T12:00:00Z",
+		LastSeenVersion: "v0.122.0",
+	}
+
+	if err := ds.SaveUpdateCheckState(state); err != nil {
+		t.Fatalf("SaveUpdateCheckState failed: %v", err)
+	}
+
+	loaded, err := ds.GetUpdateCheckState()
+	if err != nil {
+		t.Fatalf("GetUpdateCheckState failed: %v", err)
+	}
+
+	if loaded != state {
+		t.Errorf("Expected %+v, got %+v", state, loaded)
+	}
+}
+
 // TestRecordActivity_EmptyKindReturnsNilNotError verifies GetActivityRecords
 // for a kind that was never recorded returns an empty, non-error result —
 // the "nothing recorded yet" case, not a failure.