acp: scaffold zeph-acp crate with feature gate

[bug-ops]

Parent: #762

Task

Bootstrap zeph-acp crate and wire it into the workspace.

Acceptance criteria

• crates/zeph-acp/Cargo.toml with deps: agent-client-protocol, tokio, serde, serde_json, thiserror, tracing, zeph-core
• Feature gate in root Cargo.toml: acp = ["dep:zeph-acp"]
• AcpError enum in error.rs (thiserror)
• Empty lib.rs with module stubs and re-exports
• CLI argument --acp behind #[cfg(feature = "acp")] in main.rs
• Compiles with cargo build (no-op) and cargo build --features acp