feat: use ACP session/list as session-history source of truth

Extracted from #39. Gate on the `listSessions` capability from initialize; SessionManager keeps an in-memory list from `session/list` sorted by updatedAt; sessionStorage shrinks to an active-session pointer; client-side title generation is removed (titles come from goose).

This is the only type-breaking slice (SessionEntry.updatedAt, SessionStorageData drops the sessions array, AgentCapabilities.listSessions). The issue must include the migration story #39 lacks: clean up the legacy `goose.sessions.v1` globalState key, decide fallback persistence for agents without `session/list` (as written they regress to memory-only history), and bump/handle schemaVersion.

**Size**: M-L | **Depends on**: #60 (or hand-rolled types) | **Tier**: 2