fix: make the self-repo git guard Windows-only

The live-checkout git mutation guard blocked history-rewriting git ops
(checkout, reset --hard, rebase, cherry-pick, ...) in the running source
checkout and its worktrees on every platform. The hazard it protects
against is only real on Windows, where NTFS locks loaded module files and
an in-place rewrite can corrupt the running process. On POSIX, open file
handles pin the old inodes, so a checkout swap under a running process is
safe, and the guard mostly taxed normal dev/salvage workflows with clone
workarounds.

- tools/self_repo_guard.py: add guard_active() -> os.name == "nt"
- tools/terminal_tool.py: consult guard_active() before running the
  detector; detector logic and block message unchanged for Windows
- tests: wiring tests force the guard on; new tests cover the POSIX
  pass-through and the platform predicate

tests/tools/test_terminal_self_repo_guard.py

@@ -32,10 +32,12 @@ def repo(tmp_path):
     return root.resolve()
 
 
-def _run(command, config, monkeypatch, repo_root, session_cwds=None, **kwargs):
+def _run(command, config, monkeypatch, repo_root, session_cwds=None,
+         guard_on=True, **kwargs):
     from tools.terminal_tool import terminal_tool
 
     monkeypatch.setattr(self_repo_guard, "get_running_source_root", lambda: repo_root)
+    monkeypatch.setattr(self_repo_guard, "guard_active", lambda: guard_on)
     mock_env = MagicMock()
     mock_env.execute.return_value = {"output": "ok", "returncode": 0}
     mock_env.cwd = config["cwd"]
@@ -116,3 +118,19 @@ class TestSelfRepoGuardWiring:
         result, env = _run("git checkout main", config, monkeypatch, None)
         assert result.get("status") != "blocked"
         env.execute.assert_called_once()
+
+    def test_guard_inactive_passes_through(self, repo, monkeypatch):
+        """POSIX (guard_active() False): mutations in the source repo run."""
+        config = _make_env_config(cwd=str(repo))
+        result, env = _run(
+            "git reset --hard origin/main", config, monkeypatch, repo,
+            guard_on=False,
+        )
+        assert result.get("status") != "blocked"
+        env.execute.assert_called_once()
+
+    def test_guard_active_matches_platform(self):
+        """guard_active() is True exactly on Windows."""
+        import os
+
+        assert self_repo_guard.guard_active() == (os.name == "nt")

tools/self_repo_guard.py

@@ -695,6 +695,19 @@ def _find_mutation(command: str, cwd: Path, root: Path, depth: int = 0) -> str |
     return None
 
 
+def guard_active() -> bool:
+    """Whether the self-repo git guard applies on this platform.
+
+    Windows-only: NTFS locks loaded .py/.pyd files and an in-place overwrite
+    of the live checkout can corrupt the running process. On POSIX, open file
+    handles keep the old inode alive, so a checkout swap under a running
+    process is safe — already-imported modules keep executing the old code
+    and the mixed-module hazard is limited to later lazy imports, which is
+    not worth blocking every git workflow for.
+    """
+    return os.name == "nt"
+
+
 def detect_self_repo_git_mutation(
     command: str,
     cwd: str | None,

tools/terminal_tool.py

@@ -2967,18 +2967,25 @@ def terminal_tool(
                     "status": "blocked"
                 }, ensure_ascii=False)
 
-        # Non-bypassable: rewriting the local checkout backing this interpreter
-        # can mix module versions. Remote backends cannot reach that checkout.
+        # Windows-only: NTFS locks loaded module files, so rewriting the local
+        # checkout backing this interpreter can corrupt the running process.
+        # POSIX keeps old inodes alive for open handles, so the guard is off
+        # there. Remote backends cannot reach that checkout.
         if env_type == "local":
-            from tools.self_repo_guard import detect_self_repo_git_mutation
+            from tools.self_repo_guard import (
+                detect_self_repo_git_mutation,
+                guard_active,
+            )
 
             guard_cwd = _resolve_command_cwd(
                 workdir=workdir,
                 default_cwd=cwd,
                 session_key=session_key,
             )
-            _self_repo_hit, _self_repo_msg = detect_self_repo_git_mutation(
-                command, guard_cwd
+            _self_repo_hit, _self_repo_msg = (
+                detect_self_repo_git_mutation(command, guard_cwd)
+                if guard_active()
+                else (False, None)
             )
             if _self_repo_hit:
                 logger.warning(