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)

[github-actions]

This issue might be a duplicate of existing issues. Please check:

• [FEATURE]: Fork/Copy sessions with full context #4596: Fork/Copy sessions with full context (addresses session forking with context preservation)
• [Feature Request] Fork session #1759: Feature Request Fork session (earlier feature request for session forking)
• /fork #728: /fork (discussion about fork functionality for long sessions)
• [FEATURE]: /branch command to create new session with compacted context #7826: /branch command to create new session with compacted context (similar session branching feature)

Feel free to ignore if none of these address your specific case.