fix(6855): the fence keeps its files where no compaction sweeps them, and builds id lists under mawk

The project a session entered now lives in grimoire-fence, which the
compaction clear never visits, and session end removes it. Arrays of ids
in a tool's input were sent as [,"3","4"] under mawk, which creates an
array element as soon as it is assigned to.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/test_project_fence.py

@@ -190,3 +190,25 @@ def test_an_unanswered_check_lets_the_act_through_and_says_so_once(tmp_path):
     ctx = json.loads(first)["hookSpecificOutput"]["additionalContext"]
     assert ctx.startswith("> Grimoire did not answer the project fence check")
     assert _run(tmp_path, 1, _edit(other / "b.py", home), home) == ""
+
+
+def test_the_entered_project_outlives_a_compaction_and_ends_with_the_session(tmp_path):
+    # A compaction loses the context, not the session: the project it entered
+    # stays its project. Session end clears it.
+    loose = tmp_path / "loose"
+    loose.mkdir()
+    _run(tmp_path, 1, {"hook_event_name": "PostToolUse", "cwd": str(loose),
+                       "tool_name": "mcp__plugin_grimoire_grimoire__enter_project",
+                       "tool_input": {"project_id": 7}}, loose)
+    entered = tmp_path / "tmp" / "grimoire-fence" / "s-fence.entered"
+    assert entered.read_text() == "7"
+
+    env = {"PATH": os.environ["PATH"], "TMPDIR": str(tmp_path / "tmp"), "HOME": str(tmp_path)}
+    hooks = HOOK.parent
+    for hook, event in (("grimoire_session_context.sh", {"source": "compact"}),
+                        ("grimoire_session_end.sh", {"reason": "other"})):
+        subprocess.run(["bash", str(hooks / hook)], input=json.dumps({"session_id": "s-fence", **event}),
+                       capture_output=True, text=True, env=env, timeout=60, check=True)
+        if hook == "grimoire_session_context.sh":
+            assert entered.exists(), "a compaction cleared the project the session entered"
+    assert not entered.exists()