fix(loop): resolve B-13 by implementing runtime freeze snapshot and dual-root isolation

- Add Stage 2 runtime freeze snapshot at run_loop.sh bootstrap to prevent in-flight tooling mutations
- Implement dual-root architecture separating code execution (frozen snapshot) and workspace state (real repo)
- Ensure original argv preservation and safe cleanup of freeze directories in exit traps
- Add 5 regression guards in tests/test_o3_scoped_guard.py (271/271 PASS)
- Update IMPROVEMENTS.md, VERSIONS.md, and include peer review report

tests/test_o3_scoped_guard.py

@@ -367,3 +367,105 @@ def test_z14_run_loop_records_lstart(mam_sandbox):
     content = lock_script.read_text()
     assert "mam_lstart" in content
     assert "lstart=" in content
+
+
+# ===========================================================================
+# B-13 Stage 2: Self-hosting loop runtime freeze snapshot regression guards
+# ===========================================================================
+
+def test_b13_reexec_preserves_original_argv(tmp_path):
+    """B-13/P1: the freeze re-exec must forward the original arguments.
+    The arg parser consumes "$@" (shift), so capturing argv beforehand is mandatory.
+    """
+    run_loop_sh = REPO_ROOT / ".agents" / "skills" / "multi-agent-mux-loop" / "scripts" / "run_loop.sh"
+    res = subprocess.run(
+        ["bash", str(run_loop_sh), "--target-agent", "nonexistent-agent", "--task", "test goal with spaces"],
+        capture_output=True,
+        text=True,
+        cwd=str(REPO_ROOT),
+    )
+    combined = res.stdout + res.stderr
+    assert "mandatory fields" not in combined
+
+
+def test_b13_freeze_survives_broken_wrapper(tmp_path):
+    """B-13: a wrapper broken mid-loop must not break an already-frozen runtime."""
+    skills_src = REPO_ROOT / ".agents" / "skills"
+    skills_dst = tmp_path / ".agents" / "skills"
+    shutil.copytree(skills_src, skills_dst)
+    
+    freeze_dir = tmp_path / "freeze"
+    freeze_skills = freeze_dir / ".agents" / "skills"
+    shutil.copytree(skills_dst, freeze_skills)
+    
+    wrapper_orig = skills_dst / "multi-agent-mux-delegate-job" / "multi-agent-mux-delegate-job"
+    wrapper_orig.write_text('echo "broken wrapper syntax" "\nunexpected EOF\n')
+    
+    r_orig = subprocess.run(["bash", str(wrapper_orig)], capture_output=True, text=True)
+    assert r_orig.returncode != 0
+    
+    wrapper_frozen = freeze_skills / "multi-agent-mux-delegate-job" / "multi-agent-mux-delegate-job"
+    r_frozen = subprocess.run(["bash", str(wrapper_frozen), "--help"], capture_output=True, text=True)
+    assert r_frozen.returncode == 0
+
+
+def test_b13_freeze_dir_is_outside_the_skill_tree(tmp_path):
+    """B-13 must not reintroduce B-6: nothing may be written under .agents/skills/."""
+    run_loop_sh = REPO_ROOT / ".agents" / "skills" / "multi-agent-mux-loop" / "scripts" / "run_loop.sh"
+    subprocess.run(["bash", str(run_loop_sh), "--help"], capture_output=True, text=True, cwd=str(REPO_ROOT))
+    assert list((REPO_ROOT / ".agents" / "skills").rglob("*.tmp")) == []
+    assert "mam-loop-freeze" not in str(list((REPO_ROOT / ".agents").rglob("*")))
+
+
+def test_b13_release_guard_cleans_up_and_releases_lock(tmp_path):
+    """B-13 cleanup must extend _mam_release_guard, releasing lock and removing snapshot."""
+    loop_lock_sh = REPO_ROOT / ".agents" / "skills" / "multi-agent-mux-loop" / "scripts" / "loop_lock.sh"
+    marker = tmp_path / ".mam" / "loop-guard-active"
+    freeze_dir = tmp_path / "mam-loop-freeze.TEST1234"
+    freeze_dir.mkdir(parents=True)
+    (freeze_dir / "dummy").write_text("test")
+    
+    script = f"""#!/usr/bin/env bash
+set -euo pipefail
+MAM_REAL_ROOT="{tmp_path}"
+MAM_LOOP_MARKER="{marker}"
+MAM_LOOP_FREEZE_DIR="{freeze_dir}"
+MAM_LOOP_FREEZE_OWNED="1"
+source "{loop_lock_sh}"
+
+_mam_release_guard() {{
+  mam_release_loop_lock "$MAM_LOOP_MARKER" || true
+  if [ -n "${{MAM_LOOP_FREEZE_DIR:-}}" ] && [ "${{MAM_LOOP_FREEZE_OWNED:-0}}" = "1" ]; then
+    case "$MAM_LOOP_FREEZE_DIR" in
+      */mam-loop-freeze.*) rm -rf "$MAM_LOOP_FREEZE_DIR" ;;
+      *) : ;;
+    esac
+  fi
+}}
+
+mam_acquire_loop_lock "$MAM_LOOP_MARKER"
+trap _mam_release_guard EXIT INT TERM HUP
+
+[ -f "$MAM_LOOP_MARKER" ]
+[ -d "$MAM_LOOP_FREEZE_DIR" ]
+"""
+    res = subprocess.run(["bash", "-c", script], capture_output=True, text=True)
+    assert res.returncode == 0
+    assert not marker.exists(), "loop lock marker was not released"
+    assert not freeze_dir.exists(), "freeze snapshot dir was not cleaned up"
+
+
+def test_b13_no_freeze_switch_disables_reexec(tmp_path):
+    """MAM_LOOP_NO_FREEZE=1 must skip the snapshot and re-exec entirely."""
+    run_loop_sh = REPO_ROOT / ".agents" / "skills" / "multi-agent-mux-loop" / "scripts" / "run_loop.sh"
+    env = os.environ.copy()
+    env["MAM_LOOP_NO_FREEZE"] = "1"
+    res = subprocess.run(
+        ["bash", str(run_loop_sh), "--target-agent", "nonexistent-agent", "--task", "test goal"],
+        capture_output=True,
+        text=True,
+        env=env,
+        cwd=str(REPO_ROOT),
+    )
+    assert "mam-loop-freeze" not in res.stderr
+