docs: sync docs with actual stack

[catomean]

Docs truth-sweep: every claim below was verified against the repo's current state before editing.

Claim → ground truth

• HANDOVER.md: "It carries 2 high-severity vulnerabilities. Next 14.2.35 … upgrade before any real handover" → the repo has been on Next 16.2.4 since the sitekit rebuild (PR refactor: rebuild on sitekit — Next 16, site as data #1, 2026-08-28). Marked the finding as done since, keeping the rehearsal record intact.
• HANDOVER.md: npx next start → pnpm exec next start (repo is on pnpm 11.25.0 per packageManager).
• docs/DEMO.md: content SSOT claimed at src/lib/content/site.ts → actual file is src/config/site-content.ts.
• docs/DEMO.md: verify described as "lint + typecheck + build" → package.json defines it as format:check && type-check && lint && test (build runs as a separate CI step).
• docs/DEMO.md: deploy documented only as the manual fleetcrown script → canonical path is push-to-main via .github/workflows/deploy.yml → fleetcrown selfhost-deploy.yml (added 2026-08-28); manual script kept as fallback.

Not touched: the dated rehearsal transcript results, Vercel/Cloudflare mentions in the client-exit procedure (deliberate — that is the client's post-transfer hosting, not ours), and *.md formatting (prettier excludes markdown by design).

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