feat: upgrade to Astro 7; harden the content schema; wire a11y linting

Amends D1 to pin the major explicitly (v7.x) rather than inherit it. The
^5.0.0 pin was recalled rather than checked and was two majors stale the day
it was written, which meant shipping a framework carrying high-severity XSS
advisories. CLAUDE.md now requires every version pin to be verified against
the registry, and R11 requires re-checking at each build-order boundary.

npm audit now reports 0 vulnerabilities, down from 16. Every Astro advisory
is cleared; the residual 10 all traced to @lhci/cli, which is removed — it
was the sole source of 7 high-severity findings, 0.15.1 is latest so there
was no clean upgrade, and it cannot run without pages or a lighthouserc.
Re-added at build step 7 with a freshly verified pin.

Content collections migrated to the Content Layer API: src/content.config.ts,
loader: glob(), z from astro/zod.

Two review passes found seven defects in the fix itself, all now closed:

- z.coerce.date() read an unquoted 20260801 as epoch milliseconds and
  yielded 1970-01-01 silently; the first replacement then accepted
  2026-13-45 as an Invalid Date and rolled 2026-02-30 over to 2026-03-02.
  Dates are now anchored, date-only, parsed as UTC and round-tripped.
- The title bound applied the SEO spec's 50-60 to the headline rather than
  the rendered <title>, which guaranteed 68-78 on every article and rejected
  all five planned launch headlines. Articles are now the documented
  exception: the headline is the <title>, no suffix.
- An article could ship an image with no alt text, or whitespace-only alt.
- Two schema comments asserted controls nothing enforced; both are now real
  refinements, each tested with a failing and a passing case.
- PRACTICE_SLUGS and PRACTICE_AREAS could drift silently; a compile-time
  check now catches both directions.
- eslint.config.js imported globals and @eslint/js undeclared, resolving by
  hoisting accident.
- scripts/deploy-local.sh claimed parity with CI while skipping npm run
  check and two credential guards — on the only path this site can ship
  today.

Accessibility linting is on (36 jsx-a11y rules) before step 1 writes the
layout. An earlier claim in §7 that none was possible was wrong twice, and
is corrected in AGENTS.md entry (t) along with the reasoning.

Opens Q30 and Q31 for two unregistered claims in src/data/site.ts.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_012XquaEq4BgWMCwUqLEyNkF

README.md

@@ -9,7 +9,7 @@ not yet proven.
 ## Quick start
 
 ```bash
-nvm use            # Node 22
+nvm use            # Node 22 LTS — the floor is in package.json engines
 npm install
 npm run dev        # http://localhost:4321
 ```
@@ -24,7 +24,6 @@ npm run dev        # http://localhost:4321
 | `npm run check` | `astro check` — type and template errors |
 | `npm run lint` | ESLint + Prettier check |
 | `npm run format` | Prettier — rewrite files in place |
-| `npm run lighthouse` | Lighthouse CI — **not yet wired**; nothing to measure until pages exist |
 | `npm run deploy` | Build and deploy from this machine — see Deployment |
 
 ## Before you contribute