fix(docs-check): validate local Markdown section anchors

[bketelsen]

Summary

The docs-integrity gate stripped fragments before checking a link
(target.split("#")[0]) and skipped same-document links entirely, so a link
to an existing Markdown file with a stale or misspelled #anchor still counted
toward link_integrity: 1.000. AGENTS.md requires a touched doc to keep both
its targets and its section anchors valid; only the first half was enforced,
so a heading rename could silently break navigation with the gate still green.

scripts/check-docs.mjs now resolves a local fragment against the headings of
the resolved target — for same-document links against the source file itself:

• GitHub-style heading slugs (lowercase, punctuation dropped, spaces to
  hyphens), including the -1/-2 suffixes for duplicate headings.
• Headings inside fenced code blocks are stripped, as link scanning already
  does; explicit <a id="…"> / <a name="…"> anchors are honored.
• Fragments are percent-decoded and case-folded before matching.
• Only Markdown targets are anchor-checked, so a fragment on a non-.md file
  is left alone.

Path resolution, skill-tree containment, index coverage, symlink resolution,
and the pinned-heading check are untouched. Precedence is preserved: an
unresolvable path is still reported as does not resolve, never as a missing
anchor.

Resolves the gap reported by the Snowcat quality-gap-discovery root
7a7579ed-5cde-4bca-bbf5-86bd51848e0e (implementation item
ae87cf94-6fee-4b69-b7e3-f572ed0fd32d). No issue to close.

Risk tier

Risk tier: 1 — repository-local CI gate script and its hermetic tests; no
workflow, no published contract, no runtime or organization data touched.

Docs housekeeping

No docs touched — the change is limited to the gate and its tests, so the
docs-housekeeping rows do not apply.

Verification

• node scripts/check-docs.mjs green

ok   docs_index_coverage: 1.000 (required 1)
ok   link_integrity: 1.000 (required 1)
ok   symlink_resolution: 1.000 (required 1)
checked: 58 docs, 429 links, 10 symlinks

• make check (= npm run verify: check:docs, check:organization,
  check:scripts, npm test) green — 43/43 tests pass, up from 39, on the
  Node 22.23.2 pinned by mise.toml

# tests 43
# pass 43
# fail 0

• The two new negative tests fail against the unpatched gate (checked by
  reverting scripts/check-docs.mjs alone and re-running the suite):

not ok 7 - docs gate rejects a same-document link to a nonexistent section anchor
not ok 8 - docs gate rejects a cross-document link to a nonexistent section anchor

• Behavioral spot-check on the repository's one real anchored link — the
  valid docs/plans/0002-org-portfolio-roadmap.md -> 0001-docs-shape-rollout.md#phase-4--supply-chain-cleanup passes, and
  pointing it at #phase-9--nope fails with the source link and target
  named:

FAIL link: docs/plans/0002-org-portfolio-roadmap.md -> 0001-docs-shape-rollout.md#phase-9--nope has no matching section anchor in docs/plans/0001-docs-shape-rollout.md
FAIL link_integrity: 0.998 (required 1)

• Scaffold changed? — not changed
• Workflows changed? — not changed