feat(acp): upgrade agent-client-protocol SDK to 0.12.1

[bug-ops]

Description

Zeph currently uses agent-client-protocol 0.11.1 / schema 0.12.0. Upstream is now SDK 0.12.1 / schema 0.13.2. The upgrade stabilizes session lifecycle methods and input schemas that Zeph needs for production use.

Spec

See specs/013-acp/spec.md gap tracker item I1 (branch feat/m0/acp-spec-update).

Breaking changes between 0.11.1 → 0.12.1 (all confirmed no-impact for Zeph)

• McpAcpTransport removed — not used in Zeph
• acp_url renamed to acp_id — not used in Zeph
• tokio re-export removed — Zeph uses tokio directly

Acceptance criteria

• agent-client-protocol in crates/zeph-acp/Cargo.toml updated to 0.12.1
• Workspace builds without errors: cargo build --features acp
• Existing ACP integration tests pass
• /agent.json protocol field updated to "acp/0.13.2"
• session/close reason field wired through
• session/resume stabilized path tested end-to-end

Suggested next step

Run /rust-agents:team-develop dependency on this issue.