docs: update AGENTS.md bot-review section to current CodeRabbit reality (#2188)

The "Bot review workflow" section documented a manual-invocation flow
that no longer matches how this repo works.

Stale claims removed:

- "Auto-reviews are disabled on this repo — both bots are triggered
  manually." CodeRabbit auto-review is enabled here; the per-repo
  disable override was lifted.
- The 5-step workflow instructing contributors to comment
  `@copilot review` on a draft, iterate until Copilot is silent, then
  flip to ready and comment `@coderabbitai review`.
- The two-row bot table pairing Copilot with drafts and CodeRabbit
  with ready PRs.

Current reality documented instead:

- CodeRabbit reviews automatically on the draft -> ready flip and on
  every subsequent push; drafts are always skipped.
- CodeRabbit commonly edits its walkthrough comment in place rather
  than posting a new one, so the absence of a new comment is not the
  absence of a review.
- A rate-limited CodeRabbit silently drops that review; commenting
  `@coderabbitai review` after the window resets is the only case
  where a manual trigger is appropriate.
- Copilot is no longer part of the workflow and should not be
  invoked. Threads from a manual invocation by someone else are
  addressed like any other reviewer feedback.

Also adds the missing AI Validation entry to the CI list: it
cross-checks changed docs Markdown against the vyos-1x source tree for
the corresponding branch and posts inline plus summary review
comments, runs only when a PR touches docs Markdown, and skips when
the required repository secrets are unavailable or the PR is a
Mergify-authored backport.

README.md's pointer to the contributor guide is updated in the same
pass to drop Copilot from the named workflow. Note that
.github/copilot-instructions.md is a symlink to AGENTS.md, so it
picks up the change automatically.

🤖 Generated by [robots](https://vyos.io)

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