fix(guard): steer live-checkout block message to disk-backed scratch clones

The guard's "use a separate worktree or temporary clone" advice sent
agents to /tmp by default. /tmp is RAM-backed tmpfs on most distros, and
parallel salvage clones each running npm ci (~1.6GB per clone) filled a
32GB tmpfs to 97% during a 15-subagent campaign, ENOSPC-ing sibling test
runs. The message now recommends `git clone --shared <root> ~/.hermes/scratch/<task>`
(honoring HERMES_HOME), warns that dependency installs belong on real
disk, and tells the agent to delete the clone once the branch is pushed.

tests/tools/test_self_repo_guard.py

@@ -354,3 +354,28 @@ class TestUnparseableCommands:
     def test_subshell_syntax_does_not_crash(self, repo):
         hit, _ = _detect("VAL=$(git rev-parse HEAD) git checkout main", repo, repo)
         assert hit is True
+
+
+class TestBlockMessageGuidance:
+    """The block message must steer agents to a disk-backed scratch clone,
+    not a bare "temporary clone" (agents defaulted to /tmp, which is tmpfs
+    on most distros — parallel salvage clones running npm ci filled a 32GB
+    tmpfs to 97% in one campaign)."""
+
+    def test_message_recommends_shared_clone_on_disk(self, repo):
+        hit, msg = _detect("git rebase origin/main", repo, repo)
+        assert hit is True
+        assert "git clone --shared" in msg
+        assert "scratch" in msg
+
+    def test_message_warns_against_tmp_for_dep_installs(self, repo):
+        hit, msg = _detect("git rebase origin/main", repo, repo)
+        assert hit is True
+        assert "tmpfs" in msg
+        assert "Delete the clone" in msg
+
+    def test_scratch_hint_honors_hermes_home(self, repo, monkeypatch):
+        monkeypatch.setenv("HERMES_HOME", "/custom/hermes-home")
+        hit, msg = _detect("git rebase origin/main", repo, repo)
+        assert hit is True
+        assert "/custom/hermes-home/scratch" in msg

tools/self_repo_guard.py

@@ -714,9 +714,22 @@ def detect_self_repo_git_mutation(
 
 
 def _block_message(operation: str, root: Path) -> str:
+    scratch = _scratch_dir_hint()
     return (
         f"Blocked: `{operation}` would rewrite Hermes's live source checkout "
         f"({root}) and can mix module versions in this running process. "
-        "Use a separate worktree or temporary clone. To change this checkout, "
-        "stop Hermes, run the command externally, then restart Hermes."
+        f"Use a separate worktree or a shared clone on real disk, e.g. "
+        f"`git clone --shared {root} {scratch}/<task>` — avoid /tmp for "
+        "clones that install node/python deps: /tmp is usually RAM-backed "
+        "tmpfs and a few dependency installs can fill it and ENOSPC other "
+        "work. Delete the clone when the branch is pushed. To change this "
+        "checkout, stop Hermes, run the command externally, then restart "
+        "Hermes."
     )
+
+
+def _scratch_dir_hint() -> str:
+    """Disk-backed scratch location suggested to agents for temporary clones."""
+    hermes_home = os.environ.get("HERMES_HOME", "").strip()
+    base = Path(hermes_home).expanduser() if hermes_home else Path.home() / ".hermes"
+    return str(base / "scratch")