ci: adopt the canonical org lint gate

[bryanfawcett]

This repo published none of the five lint / * contexts the nyuchi org
ruleset requires on the default branch, so nothing could merge.
.github/workflows/lint.yml is now the canonical thin caller for
nyuchi/.github/.github/workflows/reusable-lint.yml. The job is named
lint and calls a reusable, so the checks publish as
lint / <called job> — exactly the five required strings. A matrix job
would publish lint (actionlint) instead and satisfy nothing.

Config files added, byte-identical to nyuchi/.github: .prettierrc,
.prettierignore, .markdownlint.jsonc, .yamllint.yaml, .editorconfig.

actionlint, JSON validity and yamllint were already clean. markdownlint
found 305 issues across 8 files; the blank-line, ordered-list-prefix,
trailing-space, heading-punctuation and bare-URL findings all auto-fixed.

Hand fixes, for the two rules that do not auto-fix:

• DEVELOPMENT_INSTRUCTIONS.md, 8 x MD024 duplicate sibling headings.
  The document contains its status sections TWICE — an original pass
  and a later revised pass appended after
  "## Summary: Ready for Firebase Integration", repeating
  "Core Features & Components Status", "Testing Status",
  "Deployment Status" and five more verbatim. Nothing was merged or
  deleted: the second, later pass is suffixed " (Updated)", which is
  what it is. Anchors are unique again, so links into the second half
  no longer silently resolve to the first.
• IMPLEMENTATION_COMPLETE.md, 1 x MD036: a bold
  "Environment Configuration" paragraph promoted to a real heading at
  the level below its parent.

A note, not a change: .astro/ is committed to this repo — data-store.json
and settings.json among them. They are Astro build artefacts, so prettier
has reformatted them here and the next astro build may well rewrite
them and turn the check red. Reformatting the JSON is semantically inert
(Astro only reads the values), so this PR leaves it there rather than
widening scope, but .astro/ almost certainly wants to be gitignored and
git rm --cachedd in a follow-up.

Verified locally with CI's exact pinned versions (actionlint 1.7.12,
prettier 3.9.4, markdownlint-cli2 0.23.2, yamllint 1.38.0): all five
clean.

Co-Authored-By: Claude Opus 5 (1M context) noreply@anthropic.com