feat(maestro): add maestro-acp, the ACP client per structured session (#134)

New crate wrapping agent-client-protocol 2.1 in the client role. spawn()
resolves claude-code-acp on the PATH the child would run with, starts it
in the session cwd with the session env, runs initialize + session/new,
and returns an AcpSession holding the connection, the ACP session id,
and an unbounded channel of raw session/update notifications for #135.

initialize declares fs.readTextFile, fs.writeTextFile and terminal as
false so the adapter uses Claude Code's own tools. request_permission is
a stub that declines until #139.

terminate() sends session/cancel, waits, SIGTERMs, then SIGKILLs. The
child stays in the daemon's process group and gets SIGTERM on daemon
death via PR_SET_PDEATHSIG. CLAUDECODE is stripped from the child env:
the adapter refuses to start when it believes it is nested in another
Claude Code session, which is how a daemon launched from one behaves.

Tests run against tests/fake_acp_agent.py, a JSON-RPC-over-stdio stand-in
that logs what it receives; real_adapter_handshake is #[ignore]d.

Cargo.toml

@@ -10,6 +10,7 @@ repository = "https://git.naps.pt/naps62/maestro"
 publish = false
 
 [workspace.dependencies]
+agent-client-protocol = "2.1"
 axum = { version = "0.7", features = ["ws"] }
 chrono = { version = "0.4", default-features = false, features = ["clock", "serde"] }
 clap = { version = "4.5", features = ["derive", "env"] }
@@ -17,6 +18,7 @@ crossterm = { version = "0.28", features = ["event-stream"] }
 dirs = "5"
 futures-util = { version = "0.3", default-features = false, features = ["sink", "std"] }
 libc = "0.2"
+maestro-acp = { path = "crates/maestro-acp" }
 maestro-config = { path = "crates/maestro-config" }
 # `default-features = false` so each consumer opts into `haiku-autoname`
 # through its own feature of the same name (#131); a workspace entry that
@@ -38,6 +40,7 @@ tempfile = "3"
 thiserror = "1"
 tokio = { version = "1", features = ["macros", "rt-multi-thread", "sync", "time", "net", "io-util", "fs", "signal", "process"] }
 tokio-tungstenite = "0.21"
+tokio-util = { version = "0.7", features = ["compat"] }
 toml = "0.8"
 tracing = "0.1"
 tracing-subscriber = { version = "0.3", features = ["env-filter"] }