feat(maestro): stamp the daemon with its build commit

`GET /api/info` now carries `build` (commit hash, commit date, dirty flag)
and `instance`, a fresh uuid per daemon process. Together they let a client
name the release it is talking to and, after losing its socket, tell an
upgrade from a plain restart from a network blip.

The stamp comes from `maestro-daemon/build.rs`, which reads the git checkout
it compiles in. The Nix build has no `.git` in the crane-filtered source, so
`flake.nix` passes the flake's own revision and last-modified date through
`MAESTRO_BUILD_*` instead. Neither available leaves the fields empty.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

crates/maestro-daemon/tests/local_api.rs

@@ -91,6 +91,35 @@ async fn info_returns_daemon_identity() {
     assert!(resp["version"].is_string());
 }
 
+#[tokio::test]
+async fn info_reports_build_and_instance() {
+    // The status indicator needs a release to name and a process to compare
+    // against after a reconnect. `commit`/`commit_date` are empty when the
+    // binary was built outside a checkout, so only their presence is asserted;
+    // `instance` is a fresh uuid per daemon process.
+    let h = spawn("naps-laptop").await;
+    let resp: serde_json::Value = reqwest::get(url(h.addr, "/api/info"))
+        .await
+        .unwrap()
+        .json()
+        .await
+        .unwrap();
+    assert!(resp["build"]["commit"].is_string());
+    assert!(resp["build"]["commit_date"].is_string());
+    assert!(resp["build"]["dirty"].is_boolean());
+    let instance = resp["instance"].as_str().unwrap();
+    assert!(Uuid::parse_str(instance).is_ok(), "instance is a uuid");
+
+    // Same process, same id — a client only treats a *changed* id as a restart.
+    let again: serde_json::Value = reqwest::get(url(h.addr, "/api/info"))
+        .await
+        .unwrap()
+        .json()
+        .await
+        .unwrap();
+    assert_eq!(again["instance"].as_str().unwrap(), instance);
+}
+
 #[tokio::test]
 async fn list_sessions_starts_empty_and_reflects_creates() {
     let h = spawn("naps-laptop").await;