docs(agents): hold a PR with the do-not-merge label, not body prose (#5407)

Closes #5333

## Summary
Implements the owner decision on #5333 (2026-09-29, Option D): agents
apply the `do-not-merge` label to a superseded or must-not-merge PR
instead of relying on body prose. No `ci-status` or workflow change.

## Fix
Adds one paragraph to root `AGENTS.md` under "Open a pull request as a
draft". It tells agents to label first, explains that prose holds
nothing because `ci-status` reads only the label, and points to
`docs/conventions/loop-lane/README.md`. The repo has no `.cursor/` files
and Cursor reads root `AGENTS.md` natively
(`docs/specs/agent-doc-surfaces.md:57-58`), so this one edit also
reaches the Cursor agent.

## Verification
- `git diff origin/main...HEAD`: one file, `AGENTS.md`, 5 lines added;
branch already contains origin/main.
- No plugin files changed, so no version bump, CHANGELOG entry, or
plugin tests apply.

## Related
Refs #4027, #4144, #4670 (context in #5333); #5154 was the merge that
prompted this.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

---------

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

Author: kyle-sexton

AGENTS.md

@@ -11,6 +11,12 @@ and then marks it ready.
 Title every pull request in Conventional Commits form, `<type>[(<scope>)]: <subject>`;
 `ci-status` fails any other title.
 
+When a pull request is superseded or must not merge, apply the `do-not-merge` label (for example
+`gh pr edit <n> --add-label do-not-merge`) before writing any explanation. "Do not merge" in the body
+or a comment is not enforced by `ci-status`, which reads only the label; the babysit merge gate
+still respects a body hold and human comments, so never override one. The hold convention is in
+`docs/conventions/loop-lane/README.md`.
+
 ## When to stop and when to keep going
 
 When a step doesn't need the user's input, keep going, with status notes in the same message as