Hash-pin every external reference in deploy.yml (closes #7)

deploy.yml was the last file in the repo carrying mutable external references.
Both job container images are now pinned by digest, all three `uses:` by a full
40-hex commit SHA, and the wrangler install by exact version, each with a
version/date comment above the reference.

- build container: klakegg/hugo:ext-alpine (abandoned since 2021, mutable tag)
  replaced by the exact alpine 3.21 digest the Dockerfile already pins, with a
  pre-checkout `apk add --no-cache nodejs git tar` step, `shell: sh` as the job
  default, then script/bootstrap and script/test. One pinned base and one
  dependency list now serve both the check build and the deploy build.
- deploy container: node:20 -> node@sha256:8f693eaa... (node 20.20.2 bookworm).
- actions/checkout: v4 -> 11bd7190... (v4.2.2), the same SHA check.yml pins.
- actions/upload-artifact: -> ff15f030... (v3.2.1).
- actions/download-artifact: -> 9bc31d5c... (v3.0.2).
- wrangler: `npm install -g wrangler` -> `wrangler@4.86.0`.

Also drops the dead feat/initial-site push trigger, reindents to 4-space YAML
to match check.yml, and adds `if: github.ref_name == 'main'` to the deploy job
so it can never publish from a branch.

This is the second attempt. The first passed two adversarial reviews, merged,
and broke the deploy, because deploy.yml triggers only on push to main and so
nobody could execute what they were reviewing. This time the workflow was
temporarily triggered on the branch, with the deploy job guarded off, and
iterated against the commit-status API until the build job ran green for real.
Doing that found two independent breaks that review had not:

1. actions/upload-artifact v4 fails on this Gitea Actions instance -- artifacts
   v4 is a different wire protocol and it is not served here. Two otherwise
   identical branch jobs, one with the v4 upload step and one without, failed
   and passed respectively. The issue asked for the v3 -> v4 bump; the
   artifact actions instead stay on the v3 line, pinned by SHA, at the exact
   commits the mutable @v3 references were already resolving to. Tracked
   separately in issue 20.
2. wrangler 4.120.0 requires node >= 22 and refuses to start on the pinned
   node 20 container. `npm install` only warns about engines, so the install
   step would have passed and the deploy step would have failed. The unpinned
   command this replaces was never installing `latest` either: npm resolves a
   bare name to the newest version whose engines the running node satisfies,
   which on node 20 is 4.86.0. So 4.86.0 is what has actually been deploying
   this site, and that is what is pinned. Tracked separately in issue 21.

The temporary branch trigger and the temporary probe workflow used to bisect
this are removed in this commit; the deploy guard is deliberately kept.

Verified: make check and script/cibuild green; the build job observed green on
the branch under act_runner (commit 73f912c, "Successful in 7s"); a probe job
pair rehearsed the deploy job end to end -- same pinned node image, same pinned
download action, same pinned wrangler, real site tarball extracted -- stopping
at `wrangler pages deploy --help` instead of publishing. The real deploy job
remains unexercised: it needs CLOUDFLARE_API_TOKEN and would publish, so it can
only run on main. The main run must still be watched and the live site
confirmed.

.gitea/workflows/deploy.yml

@@ -4,10 +4,6 @@ on:
     push:
         branches:
             - main
-            # TEMPORARY: development-only trigger so the build job actually
-            # executes under act_runner before this reaches main. Removed in
-            # the final commit.
-            - pin-deploy-refs-observable
 
 jobs:
     build:
@@ -54,14 +50,14 @@ jobs:
             - name: Archive site
               run: tar -czf site.tar.gz public
 
-            # v4 does not work on this Gitea Actions instance -- it is what
-            # broke the deploy in run 25. Measured on this branch: a job
-            # identical to this one but ending in upload-artifact v4 fails,
-            # while the same job without that step passes. So this stays on
-            # the v3 line, pinned, using the node20 build of it rather than
-            # here; tracked separately. This is the exact commit the mutable
-            # `@v3` used to resolve to, i.e. the code that was deploying this
-            # site before this issue -- now pinned instead of floating.
+            # v3, not v4: artifacts v4 is a different wire protocol and this
+            # Gitea Actions instance does not serve it. That is what broke the
+            # deploy in run 25 -- measured by running two otherwise identical
+            # jobs on a branch, one ending in upload-artifact v4 (failed) and
+            # one without that step (passed). Tracked in #20. This SHA is the
+            # exact commit the mutable `@v3` used to resolve to, i.e. the code
+            # that was already deploying this site, now pinned rather than
+            # floating.
             - name: Upload artifact
               # actions/upload-artifact v3.2.1, 2026-08-09
               uses: actions/upload-artifact@ff15f0306b3f739f7b6fd43fb5d26cd321bd4de5
@@ -102,9 +98,8 @@ jobs:
             # npm picks the newest version whose engines the running node
             # satisfies, which on node 20 is exactly 4.86.0. So this pins the
             # version that has actually been deploying this site, rather than
-            # silently changing it. Bumping the container to node 22 is the
-            # alternative; it is a bigger change and is not what this issue is
-            # for.
+            # silently changing it. Moving the container to node 22 so the
+            # wrangler pin can advance is tracked in #21.
             - name: Install Wrangler
               # wrangler 4.86.0, 2026-08-09
               run: npm install -g wrangler@4.86.0