fix(im-bot): create new Claw session on IM pairing instead of resuming old one

[bobleer]

Summary

• After successful IM QR code pairing (WeChat/Telegram/Feishu), always create a new Claw session instead of resuming the most recent one
• Prevents topic drift caused by the user being unable to see prior conversation context in the IM channel
• BitFun restart auto-reconnection is unaffected — it restores the persisted BotChatState (including current_session_id) via restore_chat_state, so no duplicate sessions are created on reconnect

Changes

Modified bootstrap_im_chat_after_pairing in command_router.rs:

• Removed logic that looked up and resumed the latest Claw session via PersistenceManager
• After successful pairing, always call handle_new_session to create a fresh Claw session
• Cleaned up unused imports (PersistenceManager, PathManager, PathBuf)

Test plan

• Pair via WeChat QR code — verify a new Claw session is created (not resuming the old one)
• Pair via Feishu — verify the same behavior
• Pair via Telegram — verify the same behavior
• Restart BitFun and auto-reconnect IM — verify no new session is created, the previous session is preserved
• Send a message from the IM client — verify it is forwarded to the newly created session correctly