ci(backport): mark not-maintained-on-stable list as exhaustive and require git verification (#2051)

* ci(backport): mark not-maintained-on-stable list as exhaustive and require git verification

The backport workflow's AI decision prompt only listed `docs/` (outside
`docs/content/`) and `skills/` as paths not maintained on `stable`, but
didn't flag that list as exhaustive. On at least one commit, the AI
generalized the pattern and incorrectly claimed `tarballs/` was also
not maintained on `stable` (it is) — likely because the commit subject
mentioned "preview tarball" and the changed files included a docs
preview smoke check workflow.

Tighten the prompt to:
- explicitly mark the not-maintained list as exhaustive;
- instruct the AI to run `git ls-tree origin/stable -- <path>` to
  verify any other path it wants to cite as main-only;
- warn it not to infer main-only-ness from suggestive names like
  "docs", "preview", "tarball", or "workflow".

* ci(backport): link to the backport job run in posted comments and PR body

Each of the comments the backport workflow posts (no-backport,
backport-created, conflict-failure) and the body of the backport PR
it opens now includes a link to the GitHub Actions run that produced
the decision. Makes it easy to jump from the comment straight to the
opencode output, prompt, and AI reasoning when the AI gets a call wrong.