Extract and test claude-code-review's self-review-skip notice shell (no CI coverage for the collapse step's matching contract)

[d-morrison]

claude-code-review.yml's "Post a notice that self-review was skipped" step
(added in #440, closing #434) carries about 30 lines of inline shell with no
CI coverage. Every comparable piece of shell in this repo has been extracted
to scripts/ plus a composite action with an offline test suite --- see
classify-push-failure.sh, detect-review-request.sh, sum-costs.sh,
build-reviewer-args.sh, select-existing-issue.sh. This one is the
exception.

What is untested

The step decides two things, and both were verified only by an ad-hoc stub
during #440's review rounds, not by anything that runs again:

1. Whether the notice posted, recorded as posted=true/posted=false on
   the skip-notice step output.
2. The body assembly --- a printf block whose output has to stay valid
   GitHub alert markup, and which must keep carrying an actions/runs/<id>
   link, because the collapse step matches comments on exactly that.

Why the second one is the load-bearing half

The posted output gates the collapse step (#440's round-2 finding 1). If the
body ever loses its run-URL link, the collapse step silently stops matching the
notice, and a PR accumulates one notice per push with nothing folding them ---
a regression with no failing check anywhere, since nothing asserts the link is
present.

That is a decidable condition over a string, which is precisely the shape
algorithmatize-checks says should not rest on a reviewer noticing.

Suggested shape

Follow the report-push-failure precedent, which is the closest analogue (it
also assembles a comment body and posts it, and has both an offline table test
and a dry-run e2e call from _selftest.yml):

• scripts/build-self-review-skip-notice.sh --- takes the workflow path and
  run URL, writes the body. Testable with no network.
• A test asserting the body contains the run URL in the form the collapse
  step's own regex (actions/runs/(?<r>[0-9]+)) matches --- ideally applying
  that literal regex, so the two cannot drift apart.
• A dry-run input on the composite so _selftest.yml can exercise the real
  uses: path, per the open-failure-issue precedent.

Note the usual bootstrapping gap if this becomes a composite: its first @v2
caller cannot land in the same PR that introduces it.

(Raised by me in #440's PR body and left as an open offer; the maintainer
merged without taking it, so filing rather than leaving it in a merged PR's
description.)