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

Start by reviewing the SessionManager and sessionStorage modules to understand the current session handling. Examine the `listSessions` capability from the initialize flow and the types in #60. The work involves migrating from the legacy `goose.sessions.v1` globalState key, updating schemaVersion, and ensuring fallback for agents without `session/list`. Testing will require verifying session history persists correctly from the ACP session/list source.

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