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.

cli.py

@@ -940,6 +940,18 @@ def _run_state_db_auto_maintenance(session_db) -> None:
         except Exception as _prune_exc:
             logger.debug("Ghost session prune skipped: %s", _prune_exc)
 
+        # One-time finalize of orphaned compression continuations (#20001).
+        try:
+            if not session_db.get_meta("orphaned_compression_finalize_v1"):
+                finalized = session_db.finalize_orphaned_compression_sessions()
+                session_db.set_meta("orphaned_compression_finalize_v1", "1")
+                if finalized:
+                    logger.info(
+                        "Finalized %d orphaned compression sessions", finalized
+                    )
+        except Exception as _finalize_exc:
+            logger.debug("Orphan compression finalize skipped: %s", _finalize_exc)
+
         cfg = (_load_full_config().get("sessions") or {})
         if not cfg.get("auto_prune", False):
             return