fix: resolve lazy session creation regressions (#18370 fallout) (#20363)

Fix three regressions introduced by PR #18370 (lazy session creation):

1. _finalize_session() uses stale session_key after compression (#20001)
2. session_key not synced after auto-compression in run_conversation (#20001)
3. pending_title ValueError leaves title wedged forever (#19029)
4. Gateway silently swallows null responses when agent did work (#18765)
5. One-time cleanup for accumulated ghost compression continuations (#20001)

Changes:
- tui_gateway/server.py: _finalize_session() now uses agent.session_id
  (falls back to session_key when agent is None). Refactor
  _sync_session_key_after_compress() with clear_pending_title and
  restart_slash_worker policy flags. Call it post-run_conversation()
  to sync session_key after auto-compression. Add ValueError handler
  to pending_title flush.
- gateway/run.py: Extract _normalize_empty_agent_response() helper that
  consolidates failed/partial/null response handling. Surfaces user-facing
  error when agent did work (api_calls > 0) but returned no text.
- hermes_state.py: Add finalize_orphaned_compression_sessions() — marks
  ghost continuation sessions as ended (non-destructive, preserves data).
- cli.py: One-time startup migration for orphaned compression sessions.

Test changes:
- tests/test_tui_gateway_server.py: Update pending_title ValueError test
  for post-#18370 architecture (title applied post-message, not at create).
- tests/test_lazy_session_regressions.py: 14 new regression tests covering
  all fixed paths.

hermes_state.py

@@ -718,6 +718,45 @@ class SessionDB:
                 self._remove_session_files(sessions_dir, sid)
         return len(removed_ids)
 
+    def finalize_orphaned_compression_sessions(self) -> int:
+        """Mark orphaned compression continuation sessions as ended.
+
+        Targets child sessions that were never finalized: parent is ended
+        with reason='compression', child has messages but no end_reason/ended_at
+        and api_call_count=0.  Non-destructive: preserves all messages and sets
+        end_reason='orphaned_compression'.  Fix for #20001.
+        """
+        cutoff = time.time() - 604800  # 7 days
+
+        def _do(conn):
+            now = time.time()
+            result = conn.execute(
+                """
+                UPDATE sessions
+                SET ended_at = ?,
+                    end_reason = 'orphaned_compression'
+                WHERE api_call_count = 0
+                  AND end_reason IS NULL
+                  AND ended_at IS NULL
+                  AND started_at < ?
+                  AND parent_session_id IS NOT NULL
+                  AND EXISTS (
+                      SELECT 1 FROM sessions p
+                      WHERE p.id = sessions.parent_session_id
+                        AND p.end_reason = 'compression'
+                        AND p.ended_at IS NOT NULL
+                  )
+                  AND EXISTS (
+                      SELECT 1 FROM messages m
+                      WHERE m.session_id = sessions.id
+                  )
+                """,
+                (now, cutoff),
+            )
+            return result.rowcount
+
+        return self._execute_write(_do) or 0
+
     def get_session(self, session_id: str) -> Optional[Dict[str, Any]]:
         """Get a session by ID."""
         with self._lock: