ci(docs): set include-hidden-files=true to preserve .nojekyll for Pages

QA-driven follow-up to b1ce91c (the bulk Node-20 sweep). The QA
reviewer flagged that Pages action release notes weren't cited
in the same detail as actions/* bumps; pulling them now revealed:

  upload-pages-artifact v4.0.0 release notes:
  > "Potentially breaking change: hidden files (specifically
  >  dotfiles) will not be included in the artifact"

MkDocs Material's `mkdocs build` writes `.nojekyll` to `site/`
so GitHub Pages skips Jekyll processing on the published artifact
— without `.nojekyll`, any path beginning with `_` (which MkDocs
Material uses for `_static/`, `_sources/`, etc.) gets 404'd by
GitHub Pages's Jekyll filter, breaking large parts of the site.

The new `include-hidden-files: true` input added in
upload-pages-artifact v5.0.0 (per its release notes) restores
the v3 behavior of including dotfiles. Setting it explicitly
makes the contract intent visible in the workflow.

Why this is "safe today, broken when public":
- docs.yml deploy job is guarded by `if: ... repository.private
  == false`. Repo is currently private (Phase 3.4 hasn't flipped
  yet), so Pages deploy is a no-op today.
- When Phase 3.4 flips public, this step starts firing — and
  WITHOUT `include-hidden-files: true`, the deploy would silently
  exclude `.nojekyll` and the published site would have broken
  asset paths.
- Catching this BEFORE the flip is cheap. Catching it AFTER would
  show up as customer-facing 404s on docs.

No DECISION_LOG entry — this is a defensive parameter add on top
of the bulk sweep, not a new architectural decision. The MEDIUM-
severity QA finding from the b1ce91c review is closed by this
commit.

doc-drift OK. YAML parses clean.

Author: RelativisticJet

.github/workflows/docs.yml

@@ -149,6 +149,15 @@ jobs:
         uses: actions/upload-pages-artifact@v5
         with:
           path: site/
+          # upload-pages-artifact v4.0.0 introduced a "hidden files are
+          # excluded by default" change. MkDocs Material writes
+          # `.nojekyll` to `site/` so GitHub Pages skips Jekyll
+          # processing — without that file, GitHub Pages would 404 on
+          # mkdocs-emitted paths starting with `_` (assets/_static
+          # paths, etc.). Setting include-hidden-files: true preserves
+          # v3 behavior and ensures the deploy is correct when Phase
+          # 3.4 flips the repo public and this step actually fires.
+          include-hidden-files: true
 
       - name: Deploy to GitHub Pages
         id: deployment