feat(acp): add persisted session list and load support

[giuliastro]

Summary

Add standard ACP persisted-session discovery and loading to Prime Agent.

• advertise sessionCapabilities.list and loadSession
• implement session/list over the existing AgentConnection.listSavedSessions() registry
• support optional cwd filtering and opaque cursor pagination
• implement session/load via AgentConnection.switchSession()
• replay persisted user/assistant text as standard ACP session/update chunks
• keep the existing one-active-session-per-connection invariant
• add protocol-level tests and update ACP documentation

Motivation

ACP session/list is now stable and gives clients a standard way to discover agent-owned persisted sessions. Prime Agent already exposes the required saved-session primitives internally (listSavedSessions and switchSession), so supporting this at the ACP boundary avoids client-specific session registries and adapters.

This is useful for editors, remote clients and other ACP consumers that want to browse and reopen Prime Agent sessions without depending on Prime-specific filesystem knowledge.

Behavior

session/list returns persisted Prime Agent session IDs, cwd, title and modification timestamp. Results are filtered by cwd when supplied and paginated with an opaque cursor.

session/load resolves the ACP sessionId through Prime Agent's saved-session registry, switches the underlying AgentConnection, subscribes to live updates, and replays persisted user/assistant text before returning.

The ACP process still hosts only one active session at a time. Clients must session/close before creating or loading another session on the same connection.

Tests

Added acp-sessions.test.ts covering persisted-session listing/filtering and load/history replay.

Note

Add session/list and session/load support to ACP mode

• Adds a session/list handler that returns persisted sessions with optional cwd filtering and cursor-based pagination (page size 50).
• Adds a session/load handler that switches to a saved session by ID and replays prior user/assistant text messages as session/update notifications before returning.
• Only one active session is allowed per connection; session/new or session/load is refused while a session is active.
• The initialize response now advertises loadSession: true and the list session capability.
• Behavioral Change: non-text content from prior messages is omitted during replay on session/load.

^{Macroscope summarized c172efb.}