feat(acp): add session/list and session/fork support

[remorses]

Add support for ACP draft/unstable methods:

• session/list - List existing sessions with cursor-based pagination
• session/fork - Fork an existing session to create a new independent session

These are part of the ACP draft spec:

• https://agentclientprotocol.com/protocol/draft/schema#session-list
• https://agentclientprotocol.com/protocol/draft/schema#session-fork

Use cases:

• session/list: Clients can discover and load previous sessions
• session/fork: Create branches from existing conversations (e.g., for generating summaries without affecting original)