fix: lazy session creation — defer DB row until first message (#18370)

Prevents ghost sessions from accumulating in state.db when the TUI/web
dashboard is opened and closed without sending a message.

Changes:
- run_agent.py: Add _ensure_db_session() gate method, called at
  run_conversation() entry. Remove eager create_session() from __init__.
  Handle compression rotation flag correctly.
- tui_gateway/server.py: Remove eager db.create_session() in
  _start_agent_build(). Add post-first-message pending_title re-apply.
- hermes_state.py: Extract _insert_session_row() shared helper (DRY).
  Add prune_empty_ghost_sessions() for one-time migration.
- cli.py: One-time ghost session prune on startup. Fix _pending_title
  to call _ensure_db_session() before set_session_title().
- hermes_cli/main.py: Guard TUI exit summary on message_count > 0.
- tests: Update test_860_dedup to call _ensure_db_session() before
  direct _flush_messages_to_session_db() calls.

Closes: ghost session clutter in hermes sessions list and web dashboard.

tests/run_agent/test_860_dedup.py

@@ -38,6 +38,8 @@ class TestFlushDeduplication:
                 skip_context_files=True,
                 skip_memory=True,
             )
+        # Simulate lazy session creation (normally done by run_conversation)
+        agent._ensure_db_session()
         return agent
 
     def test_flush_writes_only_new_messages(self):