fix: #160 — add list_sessions, session_exists, delete_session to session_store

- list_sessions(directory=None) -> list[str]: enumerate stored session IDs
- session_exists(session_id, directory=None) -> bool: check existence without FileNotFoundError
- delete_session(session_id, directory=None) -> bool: unlink a session file
- load_session now raises typed SessionNotFoundError (subclass of KeyError) instead of FileNotFoundError
- Claws can now manage session lifecycle without reaching past the module to glob filesystem

Closes ROADMAP #160. Acceptance: claw can call list_sessions(), session_exists(id), delete_session(id) without importing Path or knowing .port_sessions/<id>.json layout.

src/session_store.py

@@ -26,10 +26,65 @@ def save_session(session: StoredSession, directory: Path | None = None) -> Path:
 
 def load_session(session_id: str, directory: Path | None = None) -> StoredSession:
     target_dir = directory or DEFAULT_SESSION_DIR
-    data = json.loads((target_dir / f'{session_id}.json').read_text())
+    try:
+        data = json.loads((target_dir / f'{session_id}.json').read_text())
+    except FileNotFoundError:
+        raise SessionNotFoundError(f'session {session_id!r} not found in {target_dir}') from None
     return StoredSession(
         session_id=data['session_id'],
         messages=tuple(data['messages']),
         input_tokens=data['input_tokens'],
         output_tokens=data['output_tokens'],
     )
+
+
+class SessionNotFoundError(KeyError):
+    """Raised when a session does not exist in the store."""
+    pass
+
+
+def list_sessions(directory: Path | None = None) -> list[str]:
+    """List all stored session IDs in the target directory.
+    
+    Args:
+        directory: Target session directory. Defaults to DEFAULT_SESSION_DIR.
+    
+    Returns:
+        Sorted list of session IDs (JSON filenames without .json extension).
+    """
+    target_dir = directory or DEFAULT_SESSION_DIR
+    if not target_dir.exists():
+        return []
+    return sorted(p.stem for p in target_dir.glob('*.json'))
+
+
+def session_exists(session_id: str, directory: Path | None = None) -> bool:
+    """Check if a session exists without raising an error.
+    
+    Args:
+        session_id: The session ID to check.
+        directory: Target session directory. Defaults to DEFAULT_SESSION_DIR.
+    
+    Returns:
+        True if the session file exists, False otherwise.
+    """
+    target_dir = directory or DEFAULT_SESSION_DIR
+    return (target_dir / f'{session_id}.json').exists()
+
+
+def delete_session(session_id: str, directory: Path | None = None) -> bool:
+    """Delete a session file from the store.
+    
+    Args:
+        session_id: The session ID to delete.
+        directory: Target session directory. Defaults to DEFAULT_SESSION_DIR.
+    
+    Returns:
+        True if the session was deleted, False if it did not exist.
+    """
+    target_dir = directory or DEFAULT_SESSION_DIR
+    path = target_dir / f'{session_id}.json'
+    if path.exists():
+        path.unlink()
+        return True
+    return False