PR #224 fixed two stale references to removed docs by hand. The root
cause: wiki-lint only validated docs/wiki/, never the top-level
docs/*.md — so a doc could link to a removed sibling forever with
nothing to catch it.
Add check #4: scan docs/*.md for two doc-reference patterns and
verify they resolve (relative to docs/ or repo root):
a) markdown links [label](local.md) — the exact #224 bug class
b) backtick SHOUTING-CASE .md refs (e.g. `FOO-BAR.md`)
Scoped to doc-to-doc references deliberately. External URLs, anchors,
and cross-repo paths are skipped, and bare lowercase source filenames
(env.sh, build.sh — often runtime/contextual) are out of scope, so
the check has zero false positives on current main (171 pass) and
fail-closes under --strict (which CI already runs).
Calibrated by injecting fake removed-doc links: both the markdown
link and the backtick doc-name ref are detected and exit non-zero.
(Sam & Claude)
