chore(acp): track ACP v2 breaking changes (prompt lifecycle, remote transports, capabilities)

[bug-ops]

Description

The ACP team has signalled upcoming v2 breaking changes. This issue tracks them for forward-compatibility planning.

Spec

See specs/013-acp/spec.md v2 Horizon section, gap tracker item I11.

Known upcoming changes

• Prompt lifecycle messages — new message types for prompt start/end/cancel within a session
• Remote transports — ACP-native transport for connecting to remote agents (not just localhost IDE)
• Capabilities cleanup — capability negotiation API may be restructured; some capability names may be renamed

Action

• No implementation until v2 schema reaches release candidate
• Review Zeph's capability negotiation code when v2 RC drops (likely requires changes to crates/zeph-acp/src/agent_info.rs or equivalent)
• Update specs/013-acp/spec.md when v2 stabilizes

Tracking refs

• ACP repo: https://github.com/agentclientprotocol/agent-client-protocol
• Monitor releases and milestone discussions