Add backport PR skill (#93973)
### What?
Adds a new `$backport-pr` skill for backporting merged PRs from `canary`
to release branches.
### Why?
Backports follow a repeatable workflow: branch from the release branch,
cherry-pick the merged PR commit, verify, then open a PR against that
release branch. Capturing it as a skill helps Codex do that
consistently.
### How?
Adds `.agents/skills/backport-pr/SKILL.md` with inputs, git workflow,
verification guidance, and the backport PR body shape. Adds a one-line
reference in `AGENTS.md`.
### Verification
- `python3
/Users/timneutkens/.codex/skills/.system/skill-creator/scripts/quick_validate.py
.agents/skills/backport-pr`
- `pnpm prettier --with-node-modules --ignore-path .prettierignore
--write AGENTS.md .agents/skills/backport-pr/SKILL.md`
- `git diff --cached --check`
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