Add compacted_at_commit field and git commit capture during compaction

- Add compacted_at_commit field to Issue type (bd-405)
- Add database schema and migration for new field
- Create GetCurrentCommitHash() helper function
- Update ApplyCompaction to store git commit hash (bd-395)
- Update compaction calls to capture current commit
- Update tests to verify commit hash storage
- All tests passing

Amp-Thread-ID: https://ampcode.com/threads/T-5518cccb-7fc9-4dcd-ba5a-e22cd10e45d7
Co-authored-by: Amp <amp@ampcode.com>

internal/storage/sqlite/sqlite.go

@@ -87,6 +87,11 @@ func New(path string) (*SQLiteStorage, error) {
 		return nil, fmt.Errorf("failed to migrate compaction config: %w", err)
 	}
 
+	// Migrate existing databases to add compacted_at_commit column
+	if err := migrateCompactedAtCommitColumn(db); err != nil {
+		return nil, fmt.Errorf("failed to migrate compacted_at_commit column: %w", err)
+	}
+
 	return &SQLiteStorage{
 		db: db,
 	}, nil
@@ -401,6 +406,31 @@ func migrateCompactionConfig(db *sql.DB) error {
 	return nil
 }
 
+// migrateCompactedAtCommitColumn adds compacted_at_commit column to the issues table.
+// This migration is idempotent and safe to run multiple times.
+func migrateCompactedAtCommitColumn(db *sql.DB) error {
+	var columnExists bool
+	err := db.QueryRow(`
+		SELECT COUNT(*) > 0
+		FROM pragma_table_info('issues')
+		WHERE name = 'compacted_at_commit'
+	`).Scan(&columnExists)
+	if err != nil {
+		return fmt.Errorf("failed to check compacted_at_commit column: %w", err)
+	}
+
+	if columnExists {
+		return nil
+	}
+
+	_, err = db.Exec(`ALTER TABLE issues ADD COLUMN compacted_at_commit TEXT`)
+	if err != nil {
+		return fmt.Errorf("failed to add compacted_at_commit column: %w", err)
+	}
+
+	return nil
+}
+
 // getNextIDForPrefix atomically generates the next ID for a given prefix
 // Uses the issue_counters table for atomic, cross-process ID generation
 func (s *SQLiteStorage) getNextIDForPrefix(ctx context.Context, prefix string) (int, error) {
@@ -846,11 +876,12 @@ func (s *SQLiteStorage) GetIssue(ctx context.Context, id string) (*types.Issue,
 	var compactedAt sql.NullTime
 	var originalSize sql.NullInt64
 
+	var compactedAtCommit sql.NullString
 	err := s.db.QueryRowContext(ctx, `
 		SELECT id, title, description, design, acceptance_criteria, notes,
 		       status, priority, issue_type, assignee, estimated_minutes,
 		       created_at, updated_at, closed_at, external_ref,
-		       compaction_level, compacted_at, original_size
+		       compaction_level, compacted_at, compacted_at_commit, original_size
 		FROM issues
 		WHERE id = ?
 	`, id).Scan(
@@ -858,7 +889,7 @@ func (s *SQLiteStorage) GetIssue(ctx context.Context, id string) (*types.Issue,
 		&issue.AcceptanceCriteria, &issue.Notes, &issue.Status,
 		&issue.Priority, &issue.IssueType, &assignee, &estimatedMinutes,
 		&issue.CreatedAt, &issue.UpdatedAt, &closedAt, &externalRef,
-		&issue.CompactionLevel, &compactedAt, &originalSize,
+		&issue.CompactionLevel, &compactedAt, &compactedAtCommit, &originalSize,
 	)
 
 	if err == sql.ErrNoRows {
@@ -884,6 +915,9 @@ func (s *SQLiteStorage) GetIssue(ctx context.Context, id string) (*types.Issue,
 	if compactedAt.Valid {
 		issue.CompactedAt = &compactedAt.Time
 	}
+	if compactedAtCommit.Valid {
+		issue.CompactedAtCommit = &compactedAtCommit.String
+	}
 	if originalSize.Valid {
 		issue.OriginalSize = int(originalSize.Int64)
 	}