Add the offline link gate to the 18 green repos, and unpublish the ADRs in the two that have them #66

Supersedes #64, which proposed sharing the external check as a cross-repo reusable
workflow. That was retired on measurement — see its closing comment. This is the half that
survived, and it needs no shared machinery.

What

Add a blocking, offline link check to each repo's existing _checks.yml, matching what
.github runs since #65:

  links:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v6
      # --offline blocks network requests and excludes every external URL, so this gate
      # is deterministic: it fails only on a relative link or file path a diff broke.
      - name: Check local links
        uses: lycheeverse/lychee-action@v2
        with:
          args: >-
            --offline
            --no-progress
            '**/*.md'

No exclusions, no token, no schedule, no cross-repo dependency. --offline blocks all
network requests and excludes external URLs rather than erroring on them, which is what
makes it safe to block on.

Why

A relative link breaks because a diff broke it, and today nothing catches that. mkdocs build --strict validates links inside docs/ only — root Markdown, .github/ and
docs/agents/ are unchecked in every repo.

Measured across all 28 non-archived repos with the lychee container:

• 77 broken local links org-wide, 77 of 77 inside planning/, zero in any user-facing
  surface. Checked against a synthetic repo with deliberately broken links in README.md
  and docs/ to confirm the clean result is real and not a scan-scope artefact.
• 21 repos are green today. The other 7 (httpware 56, lite-bootstrap 8, compose2pod 4,
  semvertag 3, faststream-concurrent-aiokafka 3, faststream-redis-timers 2, db-retry 1) are
  broken only in planning/ history.

Scope: 18 repos

The 21 green repos, minus .github (already has it) and minus the two template repos
(fastapi-sqlalchemy-template, litestar-sqlalchemy-template — 3 Markdown files and one
checkable link each; the gate there is green but near-vacuous).

17 of the 18 follow the _checks.yml convention. chat-app has only a main.yml and
needs a different shape.

The 7 repos still carrying planning/ are deliberately not in scope. They go green as
a side effect of the convention migration that deletes planning/, so the gate should be
adopted there as part of it. Adding --exclude-path planning instead was considered and
rejected: an exclusion added for a transitional reason outlives the reason.

Also in scope: stop publishing the ADRs

Exactly two repos have ADRs — modern-di (27) and faststream-outbox (2) — and both carry
the identical not_in_nav: /adr/ block whose stated rationale was "built so their outgoing
links are validated". #65 retired that rationale here: the PR gate validates ADR links from
disk, so the ADRs move to exclude_docs and stop being publicly reachable.

Order matters: the gate lands first in those two repos, so link validation transfers
before --strict stops covering it. Both score 0 errors today, so both land green.

Decisions already taken

• --include-fragments stays off. Tested: it found 0 real problems and 5 false ones.
  lychee computes GitHub-flavoured heading slugs; MkDocs uses python-markdown's, which
  collapse punctuation differently, so an em dash, & or backticks in a heading produce a
  spurious "Cannot find fragment" for a link that works on the site. Confirmed by running
  markdown.extensions.toc.slugify against the three headings involved.
• Not a reusable workflow. Copied into each repo, matching how the org already shares
  _checks.yml and report-scheduled-failure.sh. The config is 5 static lines with no
  parameters, so there is nothing to drift.

Acceptance criteria

• The 18 repos each fail CI on a PR that introduces a broken relative link.
• No repo's gate is red on adoption.
• modern-di and faststream-outbox no longer serve /adr/ on their docs sites, and
  mkdocs build --strict still passes in both.
• chat-app is either adopted in whatever shape suits main.yml, or explicitly skipped.

Out of scope

• The external/weekly check in any repo but .github.
• Fixing the 77 broken planning/ links; the migration deletes them.
• The duplicate-issue defect in .github's links.yml — separate, tracked on its own.