fix(herdr): detach daemon bootstrap into own process group via python spawner

- Replace 'nohup ... server & disown' with portable Python 'Popen(..., start_new_session=True)' in lib.sh shim generator
- Isolate headless herdr server in its own SID/PGID to protect against parent process-group signal broadcast (SIGTERM/SIGINT)
- Add regression test test_h26_daemon_spawn_uses_process_group_detachment to verify child PGID detachment (Closes #3 Item 1)

.agents/skills/lib.sh

@@ -208,9 +208,21 @@ except Exception:
     # Headless bootstrap avoids herdr's "nested herdr is disabled" guard that
     # blocks a normal interactive `herdr --session <name>` launch from inside
     # an existing herdr pane (which is how MAM's own agents usually run).
-    nohup "$REAL_HERDR" --session "$_MAM_SESSION" server >/dev/null 2>&1 &
-    _mam_server_pid=$!
-    disown 2>/dev/null || true
+    # ISSUE-3: plain `nohup ... & disown` only ignores SIGHUP and drops bash's
+    # own job-table tracking - it does NOT create a new process group/session,
+    # so the daemon stays in the caller's PGID and dies when a process-group
+    # signal (SIGTERM/SIGINT from a test runner, timeout wrapper, or parent
+    # supervisor teardown) is broadcast to that group. setsid(1) is not
+    # installed by default on macOS, so spawn via Python's
+    # start_new_session=True (os.setsid in the child) instead - portable
+    # across macOS and Linux, and lib_py already depends on Python.
+    _mam_server_pid=$(python3 -c '
+import subprocess, sys
+p = subprocess.Popen(sys.argv[1:], start_new_session=True,
+                      stdout=subprocess.DEVNULL, stderr=subprocess.DEVNULL,
+                      stdin=subprocess.DEVNULL)
+print(p.pid)
+' "$REAL_HERDR" --session "$_MAM_SESSION" server)
     for _mam_wait_i in $(seq 1 40); do
       [ -S "${HOME:-$HOME_DIR}/.config/herdr/sessions/$_MAM_SESSION/herdr.sock" ] && break
       kill -0 "$_mam_server_pid" 2>/dev/null || break