feat(acp): track MCP-over-ACP transport (unstable)

[bug-ops]

Description

ACP SDK 0.12.x introduced McpAcpTransport for tunnelling MCP over ACP channels (subsequently renamed/restructured in 0.12.0). This feature is unstable and subject to breaking changes. This issue is for tracking, not implementation.

Spec

See specs/013-acp/spec.md MCP-over-ACP section, gap tracker item I8.

Current status

• McpAcpTransport was removed in SDK 0.12.0 (the transport API was restructured)
• The feature is likely re-introduced under a different API in a future stable release
• Zeph does not use the old API (confirmed by grep)

Action

• Monitor upstream for stabilization
• When stable: evaluate whether Zeph's existing MCP client (zeph-mcp) should use ACP-channel transport for IDE-embedded sessions
• No implementation until upstream stabilizes

Tracking refs

• ACP repo: https://github.com/agentclientprotocol/agent-client-protocol
• Relevant SDK changelog entries: 0.12.0 breaking changes