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.

TODO.md

@@ -14,7 +14,8 @@ pre-1.0
 
 No git tags. The site is live and now has the scripts-to-rule-them-all scaffold
 (`Makefile`, `script/`, `Dockerfile`, `check.yml`); still missing `LICENSE` and
-policy files.
+policy files. Every external reference in the repo is now pinned by
+cryptographic hash (or, for the wrangler CLI install, an exact version).
 
 # Next Step
 
@@ -24,6 +25,21 @@ Update `README.md` accordingly.
 
 # Completed Steps
 
+- 2026-08-09: hash-pinned every external reference in
+  `.gitea/workflows/deploy.yml` (closes #7): both job container images are
+  pinned by digest, all three `uses:` are pinned by 40-hex commit SHA, and the
+  wrangler install is pinned to an exact version. The abandoned
+  `klakegg/hugo:ext-alpine` image is gone: the build job now runs on the same
+  pinned `alpine` digest the `Dockerfile` uses, with a pre-checkout
+  `apk add nodejs git tar` step (the Actions runner needs `node` inside the job
+  container to execute JavaScript actions), an explicit `shell: sh` default,
+  then `script/bootstrap` and `script/test`. The `deploy` job is guarded with
+  `if: github.ref_name == 'main'` so it can never publish from a branch. Also
+  dropped the dead `feat/initial-site` push trigger and reindented the file to
+  4-space YAML to match `check.yml`. This is the second attempt; the first broke
+  the deploy and was reverted, so this one was verified by temporarily
+  triggering the workflow on the PR branch and iterating until the `build` job
+  ran green for real
 - 2026-07-25: added the scripts-to-rule-them-all scaffold (closes #4): `script/`
   entrypoints, `Makefile` shims, a Hugo `Dockerfile` (sha256-pinned alpine) plus
   `.dockerignore` that runs `make check`, `.gitea/workflows/check.yml` running
@@ -40,9 +56,11 @@ Update `README.md` accordingly.
 
 # Future Steps
 
-- Pin the images and actions in `deploy.yml` by sha256
-  (`klakegg/hugo:ext-alpine`, `node:20`, `actions/checkout`,
-  `upload`/`download-artifact` are all unpinned)
+- Move the artifact actions to v4 once this Gitea Actions instance serves the v4
+  artifact protocol; they are pinned on the deprecated v3 line because v4 fails
+  here (#20)
+- Move the deploy container to a pinned node 22 so the wrangler pin can advance
+  past 4.86.0 (#21)
 - Rework README.md into the standard sections: Description, Getting Started,
   Rationale, Design, TODO, License, Author (currently About, Contributing,
   Technical Details, License)