Plugin: add experimental Codex login relay

[huntharo]

Summary

• add /codex_login to start an experimental ChatGPT login relay through a bound OpenClaw conversation
• replay pasted localhost callback URLs back into the local Codex login server and treat the callback redirect as success
• avoid hijacking ordinary inbound messages while a login flow is pending, and add regression coverage for the relay flow

Validation

• pnpm test
• pnpm typecheck
• manual local login relay test
• manual remote-machine login relay test

Known Gaps

• This should only be used in a pinch. The flow exposes a localhost callback URL in chat and should require an explicit risk warning/acknowledgement.
• We should verify the conversation is appropriately private before starting login. In practice this should not run in a group/channel with anyone beyond the user and the bot.
• The current flow requires an existing bound conversation. We may want a dedicated temporary login-binding mode instead.
• The auth URL message and pasted callback URL should be cleaned up after login completes or fails.
• We likely want explicit channel capability checks before enabling this outside tightly scoped DMs.