ci: fast docs format check for docs-only PRs (#2028)

Since #1991, docs-only PRs skip the full CI matrix, so nothing
format-checks markdown (`vp check` in cli-e2e-test only runs for code
changes). As suggested in [this
comment](#1991 (comment)),
add a fast `docs-fmt` job that runs `vp fmt --check` when `code-changed
== 'false'`.

How it stays fast (about 15s total on the runner, the check itself is
about 5s for 1,879 files):

- `setup-vp` installs the released Vite+ (`version: latest`, since the
workspace `vite-plus` version can be ahead of the registry) with
`run-install: false`.
- `vp fmt` needs the root `vite.config.ts` to load, and its runtime
`import ... from 'vite-plus'` must resolve. Instead of installing the
whole workspace (which also needs the vite/rolldown subrepo clones), the
job symlinks the released `vite-plus` package out of the global install
into `node_modules/`.
- Checks the whole repo, so root markdown (README, RFCs) is covered too,
matching the `**/*.md` part of the CI skip filter.

Non-blocking as suggested: the job is not listed in the `done` job's
needs.

Verified on this PR by temporarily lifting the skip condition (middle
commits, since ci.yml itself is a code change the job would otherwise
skip here):

- [Failing
run](https://github.com/voidzero-dev/vite-plus/actions/runs/28635962494/job/84922274235)
on an intentionally misformatted canary file
- [Passing
run](https://github.com/voidzero-dev/vite-plus/actions/runs/28636039893/job/84922566220)
after removing it

The final commit restores the `code-changed == 'false'` guard, so the
job is expected to be skipped on this PR itself.

Author: fengmk2

.github/workflows/ci.yml

@@ -59,6 +59,33 @@ jobs:
               - '!.github/workflows/deploy-docs.yml'
               - '!netlify.toml'
 
+  docs-fmt:
+    name: Docs format check
+    needs: detect-changes
+    # Docs-only changes skip the full CI matrix, so nothing else format-checks
+    # them (code changes get `vp check` in cli-e2e-test). Non-blocking:
+    # intentionally not listed in the `done` job's needs.
+    if: needs.detect-changes.outputs.code-changed == 'false'
+    runs-on: ubuntu-latest
+    steps:
+      - uses: taiki-e/checkout-action@7d1e50e93dc4fb3bba58f85018fadf77898aee8b # v1.4.2
+
+      - uses: voidzero-dev/setup-vp@35171c92dd08b67d5a9d3f2a4327800e58396f2a # v1.13.0
+        with:
+          # The workspace `vite-plus` version can be ahead of the registry, so
+          # pin the released build instead of auto-detecting from package.json.
+          version: latest
+          run-install: false
+
+      - name: Run vp fmt --check
+        run: |
+          # `vp fmt` loads the root vite.config.ts, whose `import ... from 'vite-plus'`
+          # must resolve at runtime. Link the released package out of the global
+          # install instead of installing the whole workspace.
+          mkdir -p node_modules
+          ln -s "$HOME/.vite-plus/current/node_modules/vite-plus" node_modules/vite-plus
+          vp fmt --check
+
   download-previous-rolldown-binaries:
     needs: detect-changes
     if: needs.detect-changes.outputs.code-changed == 'true'