c0re: chown per-agent state writes to agent uid:gid (#673)

hive-c0re/src/lifecycle.rs

@@ -92,6 +92,42 @@ pub fn is_manager(name: &str) -> bool {
     name == MANAGER_NAME
 }
 
+/// Read the agent user's `(uid, gid)` from the container's nixos-managed
+/// `/etc/passwd`. Returns `None` when the container hasn't been built
+/// yet, the passwd file is unparseable, or the agent user is missing
+/// (e.g. legacy pre-#658 container that still runs as root).
+///
+/// Used by `forge` + `matrix` after writing per-agent state files so
+/// the bind-mounted host file ends up readable by the agent user
+/// without waiting for the next container activation to run the chown
+/// fixup (#673).
+///
+/// Notes:
+///   - Reads the *container-local* passwd at
+///     `/var/lib/nixos-containers/<container>/etc/passwd`, not the host's.
+///     The container's user-namespace shares uids with the host (no
+///     `PrivateUsers`), so the uid is directly usable in host-side
+///     `chown(2)`.
+///   - Best-effort: caller treats `None` as "skip the chown".
+#[must_use]
+pub fn agent_uid_gid(agent_name: &str) -> Option<(u32, u32)> {
+    let container = container_name(agent_name);
+    let passwd_path = format!("/var/lib/nixos-containers/{container}/etc/passwd");
+    let content = std::fs::read_to_string(&passwd_path).ok()?;
+    for line in content.lines() {
+        let mut parts = line.split(':');
+        let user = parts.next()?;
+        if user != agent_name {
+            continue;
+        }
+        let _ = parts.next()?; // x (password placeholder)
+        let uid: u32 = parts.next()?.parse().ok()?;
+        let gid: u32 = parts.next()?.parse().ok()?;
+        return Some((uid, gid));
+    }
+    None
+}
+
 fn validate(name: &str) -> Result<()> {
     if name.is_empty() {
         bail!("agent name must not be empty");