Adds scripts/check/check-doc-links.mjs that scans every docs/**/*.md
(excluding i18n mirrors, screenshots, exported diagrams, and superpowers
plans) and verifies that every internal markdown/HTML link resolves to a
file on disk. External URLs (http/https/mailto/tel), anchor-only links,
and fenced code blocks are ignored.
Supports --report (informational, exit 0), --json (machine-readable),
and --help. Default mode exits 1 when any broken link is found, so this
can be wired as a CI gate. Initial baseline reveals ~270 pre-existing
broken links carried over from prior reorgs (i18n relative paths,
removed RFC drafts, stale screenshot refs) that FASE 9 will clean up.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
