Rebuild the site as a terminal-style agent on Astro

Replace the single Gatsby page with a dark, terminal-style agent that answers
questions about Dennis's work from one curated knowledge base. There is no
model and no network call, so answers cannot be invented. A plain "Classic
view" and an optional light theme sit one click away.

Site
- Terminal UI (React island): typed answers, tool-call lines, slash commands,
  autocomplete, history, deep links, a short /oncall game
- Content focused on what recruiters scan for; personal and employer-internal
  details are left out
- No profile photo anywhere, including the new link-preview card and icons
- Corrected facts from the old page (dates, titles, tenses, skill tiers)

Tooling
- Migrate Gatsby 5 to Astro 7 (Node 22.23+); builds in about 1 second
- Self-host fonts, so visitors make no request to third parties
- Link-preview card, favicon set, web manifest and a service-worker clean-up
  for returning visitors
- Deploy workflow builds with npm ci on Node 22 and uploads dist/
- Track package-lock.json

Quality
- Tests for routing and for markup injection from visitor input
- A content guard scans every reachable answer; the list of things that must
  never be published lives in a git-ignored local file

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

scripts/make-icons.mjs

@@ -0,0 +1,39 @@
+// Regenerates the favicon, touch icon and app icons from public/icons/icon-512.png.
+// Run it after changing the source icon: node scripts/make-icons.mjs
+
+import { writeFile } from 'node:fs/promises'
+import { fileURLToPath } from 'node:url'
+
+import sharp from 'sharp'
+
+const root = fileURLToPath(new URL('../public/', import.meta.url))
+const source = `${root}icons/icon-512.png`
+const BACKGROUND = '#0a0d12'
+
+const resize = (size) => sharp(source).resize(size, size)
+
+await resize(32).toFile(`${root}favicon-32x32.png`)
+await resize(192).toFile(`${root}icons/icon-192.png`)
+// iOS fills transparent corners with black, so flatten onto the site background.
+await resize(180)
+  .flatten({ background: BACKGROUND })
+  .toFile(`${root}apple-touch-icon.png`)
+
+const manifest = {
+  name: 'Dennis Thiessen',
+  short_name: 'D. Thiessen',
+  start_url: '/',
+  display: 'minimal-ui',
+  background_color: BACKGROUND,
+  theme_color: BACKGROUND,
+  icons: [
+    { src: '/icons/icon-192.png', sizes: '192x192', type: 'image/png' },
+    { src: '/icons/icon-512.png', sizes: '512x512', type: 'image/png' },
+  ],
+}
+await writeFile(
+  `${root}manifest.webmanifest`,
+  `${JSON.stringify(manifest, null, 2)}\n`
+)
+
+console.log('icons and manifest written to public/')