Persist sessions to external storage

// Exported from @anthropic-ai/claude-agent-sdk as
// SessionStore, SessionKey, SessionStoreEntry, SessionSummaryEntry.

type SessionKey = {
  projectKey: string;
  sessionId: string;
  subpath?: string;
};

type SessionStore = {
  // Required
  append(key: SessionKey, entries: SessionStoreEntry[]): Promise<void>;
  load(key: SessionKey): Promise<SessionStoreEntry[] | null>;

  // Optional
  listSessions?(
    projectKey: string,
  ): Promise<Array<{ sessionId: string; mtime: number }>>;
  listSessionSummaries?(projectKey: string): Promise<SessionSummaryEntry[]>;
  delete?(key: SessionKey): Promise<void>;
  listSubkeys?(key: {
    projectKey: string;
    sessionId: string;
  }): Promise<string[]>;
};

type SessionSummaryEntry = {
  sessionId: string;
  mtime: number;
  data: Record<string, unknown>;
};

# Exported from claude_agent_sdk as
# SessionStore, SessionKey, SessionStoreEntry, SessionSummaryEntry.

class SessionKey(TypedDict):
    project_key: str
    session_id: str
    subpath: NotRequired[str]