feat(loop): port run_loop.sh to Python lib_py.loop with thin wrapper and strict parser (Phase 3)

.agents/skills/multi-agent-mux-loop/scripts/run_loop.sh

@@ -104,6 +104,13 @@ if [ -n "${PLANNER_SESSION_OVERRIDE:-}" ] && [ "$PLAN_MODE" = false ]; then
   exit 1
 fi
 
+# Anchor Python interpreter before freeze so freeze child processes find the project venv (O-4)
+if [ -z "${AGENT_PYTHON_BIN:-}" ]; then
+  if [ -x "$REPO_ROOT/.venv/bin/python" ]; then
+    export AGENT_PYTHON_BIN="$REPO_ROOT/.venv/bin/python"
+  fi
+fi
+
 # --- B-13 Stage 2: freeze the runtime before the loop can be edited under us ---
 # bash keeps reading a running script from disk by byte offset, so a worker that
 # edits .agents/skills/ mid-loop can break this very file (measured: even a valid
@@ -199,6 +206,23 @@ if [ "$PLAN_TALK_TURNS" -gt 0 ] && [ "$PLAN_MODE" = false ]; then
   log_warn "--plan-talk was specified but --plan mode is not enabled. Discussion turns will be ignored."
 fi
 
+# ===========================================================================
+# Phase 3: Python lib_py.loop Thin Wrapper Delegation (MAM_LOOP_PY=1 default)
+# Set MAM_LOOP_PY=0 to restore legacy shell loop execution.
+# ===========================================================================
+if [ "${MAM_LOOP_PY:-1}" = "1" ]; then
+  _py="${DELEGATE_JOB_PYTHON:-${AGENT_PYTHON_BIN:-$(_delegate_py_bin)}}"
+  export PYTHONPATH="$MAM_REAL_ROOT/.agents/skills:$REPO_ROOT/.agents/skills:${PYTHONPATH:-}"
+  export MAM_REAL_ROOT
+  export WORKSPACE_ROOT
+  export MAM_LOOP_MARKER
+  export MAM_LOOP_FREEZE_DIR
+  export MAM_LOOP_FREEZE_OWNED
+  export PYTHONDONTWRITEBYTECODE=1
+  [ -f "$MAM_REAL_ROOT/.mam.env" ] && export MAM_ENV_FILE="$MAM_REAL_ROOT/.mam.env"
+  exec "$_py" -m lib_py.loop ${MAM_LOOP_ARGV[@]+"${MAM_LOOP_ARGV[@]}"}
+fi
+
 # Verdict must occupy the report's last non-blank line — a standalone token
 # quoted mid-report (e.g. as a formatting example) never matches (P0-2).
 has_verdict() {