fix(cli): resolve -c by true MRU session

- order session listing by computed last_active in SessionDB so callers get MRU rows directly
- keep _resolve_last_session as a single-row lookup and add regression coverage for >20 session sampling

tests/hermes_cli/test_resolve_last_session.py

@@ -0,0 +1,139 @@
+"""Verify `hermes -c` picks the session the user most recently used."""
+
+from __future__ import annotations
+
+from hermes_cli.main import _resolve_last_session
+
+
+class _FakeDB:
+    def __init__(self, rows):
+        self._rows = rows
+        self.closed = False
+
+    def search_sessions(self, source=None, limit=20, **_kw):
+        rows = [r for r in self._rows if r.get("source") == source] if source else list(self._rows)
+        rows.sort(
+            key=lambda r: float(r.get("last_active") or r.get("started_at") or 0),
+            reverse=True,
+        )
+        return rows[:limit]
+
+    def close(self):
+        self.closed = True
+
+
+def test_resolve_last_session_prefers_last_active_over_started_at(monkeypatch):
+    # `search_sessions` should return in MRU order, so -c can trust row 0.
+    rows = [
+        {
+            "id": "new_started_old_active",
+            "source": "cli",
+            "started_at": 1000.0,
+            "last_active": 100.0,
+        },
+        {
+            "id": "old_started_recently_active",
+            "source": "cli",
+            "started_at": 500.0,
+            "last_active": 999.0,
+        },
+    ]
+
+    fake_db = _FakeDB(rows)
+    monkeypatch.setattr("hermes_state.SessionDB", lambda: fake_db)
+
+    assert _resolve_last_session("cli") == "old_started_recently_active"
+    assert fake_db.closed
+
+
+def test_search_sessions_exposes_last_active_column(tmp_path, monkeypatch):
+    # End-to-end: SessionDB must surface last_active and order by MRU.
+    monkeypatch.setenv("HERMES_HOME", str(tmp_path))
+    monkeypatch.setattr("pathlib.Path.home", lambda: tmp_path)
+
+    import hermes_state
+
+    from pathlib import Path
+
+    db = hermes_state.SessionDB(db_path=Path(tmp_path / "state.db"))
+    try:
+        db.create_session("s_started_later", source="cli")
+        db.create_session("s_active_later", source="cli")
+        # Force started_at ordering so the test is deterministic regardless
+        # of how quickly the two inserts land.
+        with db._lock:
+            db._conn.execute("UPDATE sessions SET started_at=? WHERE id=?", (2000.0, "s_started_later"))
+            db._conn.execute("UPDATE sessions SET started_at=? WHERE id=?", (1000.0, "s_active_later"))
+            db._conn.commit()
+
+        db.append_message("s_active_later", role="user", content="hi")
+        with db._lock:
+            db._conn.execute(
+                "UPDATE messages SET timestamp=? WHERE session_id=?",
+                (3000.0, "s_active_later"),
+            )
+            db._conn.commit()
+
+        rows = db.search_sessions(source="cli", limit=5)
+        ids = {r["id"]: r.get("last_active") for r in rows}
+
+        assert ids["s_started_later"] == 2000.0
+        assert ids["s_active_later"] == 3000.0
+        assert rows[0]["id"] == "s_active_later"
+    finally:
+        db.close()
+
+
+def test_resolve_last_session_returns_none_when_empty(monkeypatch):
+    monkeypatch.setattr("hermes_state.SessionDB", lambda: _FakeDB([]))
+    assert _resolve_last_session("cli") is None
+
+
+def test_resolve_last_session_falls_back_to_started_at(monkeypatch):
+    # When last_active is missing entirely (legacy row), fall back to
+    # started_at so the helper still picks the newest session.
+    rows = [
+        {"id": "older", "source": "cli", "started_at": 10.0},
+        {"id": "newer", "source": "cli", "started_at": 20.0},
+    ]
+    monkeypatch.setattr("hermes_state.SessionDB", lambda: _FakeDB(rows))
+    assert _resolve_last_session("cli") == "newer"
+
+
+def test_resolve_last_session_not_limited_to_newest_started_20(tmp_path, monkeypatch):
+    # Regression: when sampling by started_at, -c could miss the true MRU if
+    # it was older than the newest 20 started sessions.
+    monkeypatch.setenv("HERMES_HOME", str(tmp_path))
+    monkeypatch.setattr("pathlib.Path.home", lambda: tmp_path)
+
+    import hermes_state
+
+    from pathlib import Path
+
+    state_db = Path(tmp_path / "state.db")
+    real_session_db = hermes_state.SessionDB
+    db = real_session_db(db_path=state_db)
+    try:
+        for i in range(25):
+            sid = f"s_{i:02d}"
+            db.create_session(sid, source="cli")
+            with db._lock:
+                db._conn.execute(
+                    "UPDATE sessions SET started_at=? WHERE id=?",
+                    (10_000.0 - i, sid),
+                )
+                db._conn.commit()
+
+        target = "s_24"
+        db.append_message(target, role="user", content="latest activity")
+        with db._lock:
+            db._conn.execute(
+                "UPDATE messages SET timestamp=? WHERE session_id=?",
+                (20_000.0, target),
+            )
+            db._conn.commit()
+    finally:
+        db.close()
+
+    monkeypatch.setattr("hermes_state.SessionDB", lambda: real_session_db(db_path=state_db))
+    assert _resolve_last_session("cli") == target