Opt new PRs out of Code Rabbit by default

[codeofdusk]

Steps to reproduce:

1. Observe Code Rabbit summaries of several recent PRs. Examples are far too numerous to list here, but some selections:

• Added the Retry Installation action to the Add-on Store. #17104: Mentions changes to the exit dialogue that aren't included in this PR.
• Update liblouis to 3.31 #17080: mentions command line and eSpeak updates that aren't included in this PR.
• Revert "Use UIA caret events in Windows Terminal (#16873)" #17048: mentions update to the Seika Notetaker driver that aren't included in this PR.

Actual behavior:

PR summaries are being generated without user prompting. These summaries suggest that unrelated (but plausible) changes are being included in PRs, which could be extremely misleading.

Expected behavior:

• No AI summaries are generated without explicit user request.
• When AI summaries are generated, only relevant changes are included. This may be very difficult given the nature of large language models.

[seanbudd]

@codeofdusk - we coincidently had a similar conversation this morning