Unit 8: Wire `docs.halos.fi` to the anchor checker

[mairas]

Target repo: halos-org/docs.halos.fi

Goal: The package gets a consumer that does not wait on PR
#47, and the HaLOS docs site gains a
check it does not have today.

Requirements: R3, R16

Dependencies: Unit 3

Files:

• Modify: pyproject.toml
• Modify: .github/workflows/ci.yml

Approach:

• Add the package as a git-tag-pinned dependency.
• The existing build job already runs uv sync and mkdocs build --strict,
  so anchor validation is one added step against the built site/ directory,
  placed after the build and before the migration-script comparison.
• Exclude print_page/ — see Unit 3. Measured on a main build, that single
  generated page accounts for all 690 broken anchors and the 36 content pages
  are clean, so the step goes green on main from the first run.
• No i18n, no translation gate, no shared-workflow call.

Test scenarios:

• Integration: a branch introducing a link to a heading that does not exist
  fails CI at the anchor step.
• Happy path: main with print_page/ excluded passes the new step. This was
  measured during planning, not assumed.
• Edge case: without the exclusion the step fails on generated output alone —
  confirm the exclusion is what makes the difference, so nobody later removes it
  believing it to be decoration.

Verification:

• CI green on main, red on a scratch branch with a deliberately broken
  fragment in a content page.

[mairas]

Merged as halos-org/docs#38.

pyproject.toml pins halos-docs-tools at v0.1.0 and the build job runs check-anchors site --exclude 'print_page/*' after the existing steps. CI green: "Checked 1905 anchor links across 36 pages, 1 excluded. All anchors resolve."

Two things worth recording. Of those 1905 links, 1890 are Material's generated permalinks and table-of-contents entries that cannot break independently; 15 are cross-page. And nothing currently makes this block a merge — the repo has no required status checks and deploy.yml publishes on push independently of CI, so the step is advisory today.