Implement database handshake protocol in RPC layer

- Add ExpectedDB field to RPC Request
- Server validates client's expected DB matches daemon's DB
- Return clear error on mismatch with both paths
- Old clients (no ExpectedDB) still work with warning
- Add Path() method to storage.Storage interface
- Tests verify cross-database connections rejected

Prevents database pollution when client connects to wrong daemon.

Amp-Thread-ID: https://ampcode.com/threads/T-c4454192-39c6-4c67-96a9-675cbfc4db92
Co-authored-by: Amp <amp@ampcode.com>

internal/storage/sqlite/sqlite.go

@@ -96,9 +96,15 @@ func New(path string) (*SQLiteStorage, error) {
 		return nil, fmt.Errorf("failed to migrate compacted_at_commit column: %w", err)
 	}
 
+	// Convert to absolute path for consistency
+	absPath, err := filepath.Abs(path)
+	if err != nil {
+		return nil, fmt.Errorf("failed to get absolute path: %w", err)
+	}
+
 	return &SQLiteStorage{
 		db:     db,
-		dbPath: path,
+		dbPath: absPath,
 	}, nil
 }
 
@@ -1862,3 +1868,8 @@ func (s *SQLiteStorage) GetIssueComments(ctx context.Context, issueID string) ([
 func (s *SQLiteStorage) Close() error {
 	return s.db.Close()
 }
+
+// Path returns the absolute path to the database file
+func (s *SQLiteStorage) Path() string {
+	return s.dbPath
+}