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

hive-c0re/src/matrix.rs

@@ -274,10 +274,25 @@ pub async fn ensure_user_for(
     std::fs::write(&path, format!("{access_token}\n"))
         .with_context(|| format!("matrix: write token to {}", path.display()))?;
     let _ = std::fs::set_permissions(&path, std::fs::Permissions::from_mode(0o600));
+    chown_to_agent(name, &path);
     tracing::info!(%name, path = %path.display(), "matrix: registered user + persisted access token");
     Ok(())
 }
 
+/// Best-effort chown `path` to the agent's container-local uid/gid.
+/// Mirrors `forge::chown_to_agent` for the matrix access-token write —
+/// closes the gap where c0re (root on host) writes a file the agent's
+/// non-root unix user then can't read until the next activation runs
+/// the harness-base.nix chown fixup (#673).
+fn chown_to_agent(name: &str, path: &Path) {
+    let Some((uid, gid)) = crate::lifecycle::agent_uid_gid(name) else {
+        return;
+    };
+    if let Err(e) = std::os::unix::fs::chown(path, Some(uid), Some(gid)) {
+        tracing::debug!(%name, path = %path.display(), error = %e, "matrix: chown to agent failed");
+    }
+}
+
 /// Register a matrix account for `name` with the supplied `password`
 /// and return the freshly-minted access token. Unlike [`ensure_user_for`],
 /// the token is **not** persisted to disk — the caller is responsible