feat(dolt): add GetChangesSinceExport and CommitExists methods (bd-ejv83)

Add versioned storage methods for incremental export support:
- GetChangesSinceExport: returns changes since a commit hash, with
  NeedsFullExport flag for invalid/GC'd commits
- CommitExists: checks if a commit hash exists, supports short prefixes

Also fixes dolt_diff syntax for embedded driver (from_ref, to_ref, table).

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

internal/storage/dolt/versioned.go

@@ -49,7 +49,8 @@ func (s *DoltStore) Diff(ctx context.Context, fromRef, toRef string) ([]*storage
 		return nil, fmt.Errorf("invalid toRef: %w", err)
 	}
 
-	// Query issue-level diffs directly
+	// Query issue-level diffs using dolt_diff table function
+	// Syntax: dolt_diff(from_ref, to_ref, 'table_name')
 	// Note: refs are validated above
 	// nolint:gosec // G201: refs validated by validateRef()
 	query := fmt.Sprintf(`
@@ -61,7 +62,7 @@ func (s *DoltStore) Diff(ctx context.Context, fromRef, toRef string) ([]*storage
 			from_description, to_description,
 			from_status, to_status,
 			from_priority, to_priority
-		FROM dolt_diff_issues('%s', '%s')
+		FROM dolt_diff('%s', '%s', 'issues')
 	`, fromRef, toRef)
 
 	rows, err := s.db.QueryContext(ctx, query)
@@ -186,3 +187,82 @@ func (s *DoltStore) GetConflicts(ctx context.Context) ([]storage.Conflict, error
 	}
 	return conflicts, nil
 }
+
+// ExportChanges represents the result of GetChangesSinceExport.
+type ExportChanges struct {
+	Entries         []*storage.DiffEntry // Changes since the export commit
+	NeedsFullExport bool                 // True if fromCommit is invalid/GC'd
+}
+
+// GetChangesSinceExport returns changes since a specific commit hash.
+// If the commit hash is invalid or has been garbage collected, it returns
+// NeedsFullExport=true to indicate a full export is required.
+func (s *DoltStore) GetChangesSinceExport(ctx context.Context, fromCommit string) (*ExportChanges, error) {
+	// Empty commit means this is the first export
+	if fromCommit == "" {
+		return &ExportChanges{NeedsFullExport: true}, nil
+	}
+
+	// Validate the ref format
+	if err := validateRef(fromCommit); err != nil {
+		return &ExportChanges{NeedsFullExport: true}, nil
+	}
+
+	// Check if the commit exists
+	exists, err := s.CommitExists(ctx, fromCommit)
+	if err != nil {
+		return nil, fmt.Errorf("failed to check commit existence: %w", err)
+	}
+	if !exists {
+		return &ExportChanges{NeedsFullExport: true}, nil
+	}
+
+	// Get current HEAD commit to check if we're already at fromCommit
+	currentCommit, err := s.GetCurrentCommit(ctx)
+	if err != nil {
+		return nil, fmt.Errorf("failed to get current commit: %w", err)
+	}
+
+	// If fromCommit equals HEAD, there are no changes.
+	// Note: This also avoids a nil pointer panic in the embedded Dolt driver
+	// when querying dolt_diff with identical from/to refs.
+	if fromCommit == currentCommit {
+		return &ExportChanges{Entries: nil}, nil
+	}
+
+	// Get the diff from that commit to HEAD
+	entries, err := s.Diff(ctx, fromCommit, "HEAD")
+	if err != nil {
+		return nil, fmt.Errorf("failed to get diff: %w", err)
+	}
+
+	return &ExportChanges{Entries: entries}, nil
+}
+
+// CommitExists checks whether a commit hash exists in the repository.
+// Returns false for empty strings, malformed input, or non-existent commits.
+func (s *DoltStore) CommitExists(ctx context.Context, commitHash string) (bool, error) {
+	// Empty string is not a valid commit
+	if commitHash == "" {
+		return false, nil
+	}
+
+	// Validate format to reject malformed input
+	if err := validateRef(commitHash); err != nil {
+		return false, nil
+	}
+
+	// Query dolt_log to check if the commit exists.
+	// Supports both full hashes and short prefixes (like git's short SHA).
+	// The exact match handles full hashes; LIKE handles prefixes.
+	var count int
+	err := s.db.QueryRowContext(ctx, `
+		SELECT COUNT(*) FROM dolt_log
+		WHERE commit_hash = ? OR commit_hash LIKE ?
+	`, commitHash, commitHash+"%").Scan(&count)
+	if err != nil {
+		return false, fmt.Errorf("failed to check commit existence: %w", err)
+	}
+
+	return count > 0, nil
+}