feat(bot): replay linked account messages

[RSO]

Summary

• Replay the original chat message after a user links their Kilo account, so they do not need to resend the request.
• Extract Chat SDK thread reconstruction and authenticated bot message processing into shared modules used by both the callback and link-account flows.
• Extend link-account tokens with signed replay context and dedupe replay attempts by token nonce.

Verification

Manual browser verification not performed; this change is in the chat-platform callback/linking flow and was validated with targeted automated checks locally.

Visual Changes

N/A

Reviewer Notes

• Replay depends on the adapter supporting fetchMessage; if unavailable or the message cannot be found, the bot posts a fallback asking the user to mention Kilo again.
• The replay path verifies the fetched message author matches the linked platform identity before processing.