feat(bot): replay GitHub mentions after account linking

[RSO]

Summary

• Threads the chat message context through the GitHub link flow so that after a new user links their account we immediately replay their original mention — matching the Slack UX instead of telling them to mention Kilo again.
• Extracts the Slack-only context store (createLinkAccountToken) into shared storeLinkAccountContext / readLinkAccountContext helpers, and adds an optional contextKey to the GitHub link token and OAuth state so the key survives the OAuth round-trip.
• Hoists reprocessLinkedMessage into lib/bot/run.ts and uses it from both the Slack /api/chat/link-account route and the GitHub OAuth callback.

Verification

• Mention Kilo from an unlinked GitHub account → click the link → complete GitHub OAuth → Kilo posts a response to the original issue/PR comment without a second mention. The callback success page now reads "Kilo is already processing your message".
• Repeat the OAuth callback (e.g. browser back + re-submit) → the context is consumed once, the second attempt does not re-trigger a bot run, and the success page falls back to "mention Kilo again".
• Legacy pre-deploy link tokens (no contextKey) still link successfully; only the replay is skipped.

Visual Changes

N/A

Reviewer Notes

• consumeLinkAccountContext is the single-use guard — we call readLinkAccountContext first so we can reconstruct the context, then consumeLinkAccountContext atomically to prevent duplicate replays from OAuth retries. The Slack route keeps its existing consume → after(reprocess) order.
• createGitHubBotLinkState changed from positional callbackPath to an options bag so we can carry contextKey. Only one callsite, updated in /github/link/route.ts.
• No new PII is persisted — the Redis context only contains already-serialized thread/message payloads that were already written by the Slack flow under the same LINK_ACCOUNT_CONTEXT_KEY_PREFIX.