ci(docs): make nav-integrity's mkdocs build --strict

[bdfinst]

Summary

• Closes the CI gap docs: hand-written docs pages have ~134 more cross-tree links that 404 on the published site #2113 found: body-links (lychee) validates the raw repo tree, not the assembled MkDocs site tree that actually gets published, so a link that 404s on the live site could still pass CI.
• nav-integrity's mkdocs build now runs with --strict, which aborts on any WARNING-level build log line — including a broken body link or drifted anchor against the assembled tree. Safe to make blocking now that docs: 60 pre-existing broken links/anchors found against the assembled site tree #2118 cleared the pre-existing backlog (merged in fix(docs): fix 72 broken links/anchors against the assembled site tree #2121).
• Applied directly on the branch (pushed by @bdfinst) since this session's credentials lack the workflow OAuth scope needed to push .github/workflows/*.yml changes itself — verified equivalent to the diff prepared in ci: make nav-integrity's mkdocs build --strict (#2118 follow-up, blocked on workflow scope) #2120: same resolved run: command, mkdocs build --strict still exits 0 clean and tests/repo/test_required_status_checks.py still passes.

Test Plan

• bash scripts/assemble-docs.sh && mkdocs build --strict --site-dir /tmp/site — exit 0, clean
• python3 -m pytest tests/repo/test_required_status_checks.py -q — passes (new step on an existing job, no new required-check registration needed)
• Previously verified (PR fix(docs): fix 72 broken links/anchors against the assembled site tree #2121's prepared commit): a deliberately reintroduced broken link fails --strict with exit 1

Closes #2120

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