fix(bot): replay original message after account linking

[RSO]

Summary

• Store the original mention context per account-link token so the bot can resume the exact message the user was trying to send.
• Reuse the normal linked-message handling flow after a successful link so Kilo processes the original mention instead of stopping after the ephemeral prompt disappears.
• Update the link prompt and success page copy to explain automatic replay while keeping the manual re-mention fallback when no pending replay exists.

Verification

• pnpm typecheck - passed
• pnpm exec eslint src/lib/bot.ts src/lib/bot/link-account.ts src/lib/bot/pending-link-replay.ts src/lib/bot/handle-linked-message.ts src/app/api/chat/link-account/route.ts - passed

Visual Changes

N/A

Reviewer Notes

• Pending replays are scoped to the clicked link token and consumed under a state-adapter lock so reopening the link does not post duplicate bot replies.
• Manual Slack end-to-end testing was not run in this environment.