test(tui_gateway): manual /compress runs under the session cwd

Invariant for the previous commit: the compaction-boundary work inside
_compress_session_history resolves the session's workspace, not the backend
process cwd, and the binding does not leak past the call. Red on origin/main.

tests/tui_gateway/test_compress_session_cwd.py

@@ -0,0 +1,36 @@
+"""Manual /compress rebuilds the system prompt on the RPC thread; it must see the session's workspace.
+
+Otherwise the rebuilt prompt records the backend process cwd, and every later resume rejects it as
+stale runtime and rebuilds it again (a second prompt-cache miss right after the compaction).
+"""
+import threading
+from unittest.mock import MagicMock
+
+
+def test_manual_compress_runs_under_the_session_cwd(tmp_path, monkeypatch):
+    from agent.runtime_cwd import resolve_agent_cwd
+    from tui_gateway.server import _compress_session_history
+
+    workspace = tmp_path / "workspace"
+    workspace.mkdir()
+    monkeypatch.delenv("TERMINAL_CWD", raising=False)
+    monkeypatch.chdir(tmp_path)
+    monkeypatch.setattr("tui_gateway.server._get_usage", lambda _agent: {})
+    history = [{"role": "user" if i % 2 == 0 else "assistant", "content": f"m{i}"} for i in range(6)]
+    seen = []
+    agent = MagicMock(_cached_system_prompt="", tools=None, session_id="sess-cwd",
+                      _pending_context_engine_compression_notification=None,
+                      _compression_skipped_due_to_lock=None)
+
+    def _fake_compress(msgs=None, *_args, **_kwargs):
+        seen.append(resolve_agent_cwd())
+        return list(msgs or history)[-2:], ""
+
+    agent._compress_context.side_effect = _fake_compress
+    session = {"agent": agent, "history_lock": threading.Lock(), "history": list(history),
+               "history_version": 1, "running": False, "session_key": "sess-cwd", "cwd": str(workspace)}
+
+    _compress_session_history(session)
+
+    assert seen == [workspace]
+    assert resolve_agent_cwd() == tmp_path  # the binding does not leak past the call