feat(Phase4): add file changes for changestats executor wiring

Files changed: CLAUDE.md, internal/api/changestats.go,
internal/executor/executor.go, internal/executor/executor_test.go,
internal/task/changestats.go (new)

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

2 files changed, 159 insertions, 3 deletions

diff --git a/internal/executor/executor.go b/internal/executor/executor.go
index 7674fe6..fd37c33 100644
--- a/internal/executor/executor.go
+++ b/internal/executor/executor.go
@@ -29,6 +29,7 @@ type Store interface {
 	UpdateTaskSummary(taskID, summary string) error
 	AppendTaskInteraction(taskID string, interaction task.Interaction) error
 	UpdateTaskAgent(id string, agent task.AgentConfig) error
+	UpdateExecutionChangestats(execID string, stats *task.Changestats) error
 }
 
 // LogPather is an optional interface runners can implement to provide the log
@@ -352,6 +353,13 @@ func (p *Pool) handleRunResult(ctx context.Context, t *task.Task, exec *storage.
 			p.logger.Error("failed to update task summary", "taskID", t.ID, "error", summaryErr)
 		}
 	}
+	if exec.StdoutPath != "" {
+		if cs := task.ParseChangestatFromFile(exec.StdoutPath); cs != nil {
+			if csErr := p.store.UpdateExecutionChangestats(exec.ID, cs); csErr != nil {
+				p.logger.Error("failed to store changestats", "execID", exec.ID, "error", csErr)
+			}
+		}
+	}
 	if updateErr := p.store.UpdateExecution(exec); updateErr != nil {
 		p.logger.Error("failed to update execution", "error", updateErr)
 	}
diff --git a/internal/executor/executor_test.go b/internal/executor/executor_test.go
index a6c4ad8..610ed3b 100644
--- a/internal/executor/executor_test.go
+++ b/internal/executor/executor_test.go
@@ -921,9 +921,13 @@ type minimalMockStore struct {
 	executions      map[string]*storage.Execution
 	stateUpdates    []struct{ id string; state task.State }
 	questionUpdates []string
-	subtasksFunc    func(parentID string) ([]*task.Task, error)
-	updateExecErr   error
-	updateStateErr  error
+	changestatCalls []struct {
+		execID string
+		stats  *task.Changestats
+	}
+	subtasksFunc   func(parentID string) ([]*task.Task, error)
+	updateExecErr  error
+	updateStateErr error
 }
 
 func newMinimalMockStore() *minimalMockStore {
@@ -977,6 +981,15 @@ func (m *minimalMockStore) AppendTaskInteraction(taskID string, _ task.Interacti
 	return nil
 }
 func (m *minimalMockStore) UpdateTaskAgent(id string, agent task.AgentConfig) error { return nil }
+func (m *minimalMockStore) UpdateExecutionChangestats(execID string, stats *task.Changestats) error {
+	m.mu.Lock()
+	m.changestatCalls = append(m.changestatCalls, struct {
+		execID string
+		stats  *task.Changestats
+	}{execID, stats})
+	m.mu.Unlock()
+	return nil
+}
 
 func (m *minimalMockStore) lastStateUpdate() (string, task.State, bool) {
 	m.mu.Lock()
@@ -1203,3 +1216,138 @@ func TestPool_SpecificAgent_PersistsToDB(t *testing.T) {
 		t.Errorf("expected agent type gemini in DB, got %q", reloaded.Agent.Type)
 	}
 }
+
+// TestExecute_ExtractAndStoreChangestats verifies that when the execution stdout
+// contains a git diff --stat summary line, the changestats are parsed and stored.
+func TestExecute_ExtractAndStoreChangestats(t *testing.T) {
+	store := testStore(t)
+	logDir := t.TempDir()
+
+	runner := &logPatherMockRunner{logDir: logDir}
+	runner.onRun = func(tk *task.Task, e *storage.Execution) error {
+		if err := os.MkdirAll(filepath.Dir(e.StdoutPath), 0755); err != nil {
+			return err
+		}
+		content := "some output\n5 files changed, 127 insertions(+), 43 deletions(-)\n"
+		return os.WriteFile(e.StdoutPath, []byte(content), 0644)
+	}
+
+	runners := map[string]Runner{"claude": runner}
+	logger := slog.New(slog.NewTextHandler(os.Stderr, &slog.HandlerOptions{Level: slog.LevelError}))
+	pool := NewPool(2, runners, store, logger)
+
+	tk := makeTask("cs-extract-1")
+	store.CreateTask(tk)
+
+	if err := pool.Submit(context.Background(), tk); err != nil {
+		t.Fatalf("submit: %v", err)
+	}
+	result := <-pool.Results()
+	if result.Err != nil {
+		t.Fatalf("unexpected error: %v", result.Err)
+	}
+
+	execs, err := store.ListExecutions(tk.ID)
+	if err != nil {
+		t.Fatalf("list executions: %v", err)
+	}
+	if len(execs) == 0 {
+		t.Fatal("no executions found")
+	}
+	cs := execs[0].Changestats
+	if cs == nil {
+		t.Fatal("expected changestats to be populated, got nil")
+	}
+	if cs.FilesChanged != 5 {
+		t.Errorf("FilesChanged: want 5, got %d", cs.FilesChanged)
+	}
+	if cs.LinesAdded != 127 {
+		t.Errorf("LinesAdded: want 127, got %d", cs.LinesAdded)
+	}
+	if cs.LinesRemoved != 43 {
+		t.Errorf("LinesRemoved: want 43, got %d", cs.LinesRemoved)
+	}
+}
+
+// TestExecute_NoChangestats verifies that when execution output contains no git
+// diff stat line, changestats are not stored (remain nil).
+func TestExecute_NoChangestats(t *testing.T) {
+	store := testStore(t)
+	logDir := t.TempDir()
+
+	runner := &logPatherMockRunner{logDir: logDir}
+	runner.onRun = func(tk *task.Task, e *storage.Execution) error {
+		if err := os.MkdirAll(filepath.Dir(e.StdoutPath), 0755); err != nil {
+			return err
+		}
+		return os.WriteFile(e.StdoutPath, []byte("no git output here\n"), 0644)
+	}
+
+	runners := map[string]Runner{"claude": runner}
+	logger := slog.New(slog.NewTextHandler(os.Stderr, &slog.HandlerOptions{Level: slog.LevelError}))
+	pool := NewPool(2, runners, store, logger)
+
+	tk := makeTask("cs-none-1")
+	store.CreateTask(tk)
+
+	if err := pool.Submit(context.Background(), tk); err != nil {
+		t.Fatalf("submit: %v", err)
+	}
+	result := <-pool.Results()
+	if result.Err != nil {
+		t.Fatalf("unexpected error: %v", result.Err)
+	}
+
+	execs, err := store.ListExecutions(tk.ID)
+	if err != nil {
+		t.Fatalf("list executions: %v", err)
+	}
+	if len(execs) == 0 {
+		t.Fatal("no executions found")
+	}
+	if execs[0].Changestats != nil {
+		t.Errorf("expected changestats to be nil for output with no git stats, got %+v", execs[0].Changestats)
+	}
+}
+
+// TestExecute_MalformedChangestats verifies that malformed git-stat-like output
+// does not produce changestats (parser returns nil, nothing is stored).
+func TestExecute_MalformedChangestats(t *testing.T) {
+	store := testStore(t)
+	logDir := t.TempDir()
+
+	runner := &logPatherMockRunner{logDir: logDir}
+	runner.onRun = func(tk *task.Task, e *storage.Execution) error {
+		if err := os.MkdirAll(filepath.Dir(e.StdoutPath), 0755); err != nil {
+			return err
+		}
+		// Looks like a git stat line but doesn't match the regex.
+		return os.WriteFile(e.StdoutPath, []byte("lots of cheese changed, many insertions\n"), 0644)
+	}
+
+	runners := map[string]Runner{"claude": runner}
+	logger := slog.New(slog.NewTextHandler(os.Stderr, &slog.HandlerOptions{Level: slog.LevelError}))
+	pool := NewPool(2, runners, store, logger)
+
+	tk := makeTask("cs-malformed-1")
+	store.CreateTask(tk)
+
+	if err := pool.Submit(context.Background(), tk); err != nil {
+		t.Fatalf("submit: %v", err)
+	}
+	result := <-pool.Results()
+	if result.Err != nil {
+		t.Fatalf("unexpected error: %v", result.Err)
+	}
+
+	execs, err := store.ListExecutions(tk.ID)
+	if err != nil {
+		t.Fatalf("list executions: %v", err)
+	}
+	if len(execs) == 0 {
+		t.Fatal("no executions found")
+	}
+	if execs[0].Changestats != nil {
+		t.Errorf("expected nil changestats for malformed output, got %+v", execs[0].Changestats)
+	}
+}