fix(ci): report the docs URL from the config, not from wrangler's log

[yashau]

Follow-up to #39, fixing a defect it shipped.

What went wrong

docs-v2026.901.0 deployed correctly — https://docs.getprick.dev is live — but
published its notes as:

- url: docs.getprick.dev (custom domain)

and GitHub rejected the same string as the deployment's environment URL:

Environment URL 'docs.getprick.dev (custom domain)' is not a valid http(s)
URL, so it will not be shown as a link in the workflow graph.

wrangler-action derives deployment-url by scraping the deploy log, and for a
Worker on a custom domain the line it finds is a trigger description, not a
URL.

#39 anticipated that output going empty once workers.dev was switched off, and
guarded with || steps.site.outputs.url. That was the wrong shape of guard —
the output is not empty, it is non-empty and wrong, so the fallback never fired.

The fix

Stop consulting it. The environment URL, the job summary and the release notes
all take the hostname read out of scripts/docs.mjs, which
scripts/docs.test.mjs already ties to the routes block in
packages/docs/wrangler.jsonc and to site in its astro.config.ts. That route
is the only place a deploy can land, so it is a fact about the release rather
than a guess at one.

DEPLOY_URL becomes SITE_URL in both jobs. It no longer holds what the deploy
reported, and a name saying otherwise is how the next reader repeats this — the
comment justifying the environment hop claimed the value was "an action output
and so is not ours to trust", which stopped being true with it.

The step comment now records the observed failure mode rather than the guessed
one, with the actual string from docs-v2026.901.0 in it.

Scope

Only releases cut from this commit onward are affected — a tag push runs the
workflow as of its own commit. docs-v2026.901.0's notes have already been
corrected in place with gh release edit; the site itself was always fine.

Checks

actionlint, zizmor, lint:typos, fmt:check and the 275 script tests pass
locally, and the pre-push hook ran clippy, typecheck and both test suites green.

🤖 Generated with Claude Code