Unit 6: Move PR comment construction into the package

[mairas]

Target repo: halos-org/docs-tools

Goal: One command produces the comment body, and that body describes exactly
what the gate fails on.

Requirements: R10, R11

Dependencies: Unit 5

Files:

• Modify: src/halos_docs_tools/translation_status.py
• Modify: tests/test_translation_status.py

Approach:

• The body covers every failing entry, not only pages the pull request touched.
  Under a repo-wide gate a touched-pages comment can omit the very page that
  turned the check red.
• Keep what PR #48 established: the
  60000-character ceiling, and the fallback that drops the collapsed English
  diffs and points at the job summary when the full body exceeds it.
• Keep the <!-- translation-status --> marker so the workflow can find and
  update its own previous comment.
• The command writes a body. It does not call the GitHub API; posting stays in
  the workflow where the token lives.

Test scenarios:

• Happy path: a tree with one stale page produces a body containing that page,
  its locale, its state, the stamp to record, and the marker comment.
• Happy path: a fully current tree produces the "every translation is current"
  body rather than an empty one.
• Edge case: a body that would exceed 60000 characters is regenerated without
  diffs and carries the job-summary pointer.
• Edge case: the regenerated body is itself under the limit for a tree with
  every page stale in every locale — the case that produced the original
  HTTP 422.
• Integration: the set of pages named in the body equals the set of entries that
  make check mode exit non-zero, for the same tree.

Verification:

• Generated against PR #47's branch,
  the body stays under the ceiling and names every failing entry.