fix(chat): preserve markdown whitespace in plain text

[RSO]

Problem

When I mention a Chat SDK-powered bot on GitHub like this:

@bot
hi there!

The mention doesn't trigger any handling, because the mdastToString helper replaces all newlines with an empty string, so the logs look like this:

[chat-sdk] Checking message patterns { patternCount: 0, patterns: [], messageText: '@bothi there' }
[chat-sdk] No handlers matched message {
  threadId: 'github:RSO/chat:issue:29',
  text: '@bothi there'
}

Summary

• Preserve structural markdown whitespace when extracting normalized plain text from mdast.
• Add regression coverage for newline-separated bot mentions in core markdown extraction, GitHub issue/review comments, and Chat mention routing.
• Add a patch changeset for the behavior fix.

Testing

• pnpm check
• pnpm knip
• pnpm test:workspace
• pnpm --filter chat test
• pnpm --filter @chat-adapter/github test
• pnpm --filter chat typecheck
• pnpm --filter @chat-adapter/github typecheck

Notes

• pnpm validate was attempted and reached the full Turbo test graph, but failed on packages/integration-tests/src/replay-discord.test.ts (should skip bot's own messages in subscribed threads). Rerunning pnpm --filter @chat-adapter/integration-tests test -- src/replay-discord.test.ts passed, so this appears unrelated to the markdown whitespace change.
• .github/CONTRIBUTING.md requires signed commits. This environment has no gpg binary and no SSH signing identities loaded, so the commit in this PR is currently unsigned and may need to be re-signed before merge.

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