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>

package.json

@@ -3,38 +3,27 @@
   "description": "Personal site of Dennis Thiessen — thiessen.io",
   "version": "1.0.0",
   "author": "Dennis Thiessen",
-  "dependencies": {
-    "gatsby": "^5.12.12",
-    "gatsby-plugin-manifest": "^5.12.3",
-    "gatsby-plugin-react-helmet": "^6.12.0",
-    "gatsby-plugin-remove-serviceworker": "^1.0.0",
-    "react": "^18.2.0",
-    "react-dom": "^18.2.0",
-    "react-helmet": "^6.1.0",
-    "ajv": "^8.12.0",
-    "ajv-keywords": "^5.1.0",
-    "schema-utils": "^4.2.0"
-  },
-  "keywords": [
-    "gatsby"
-  ],
   "license": "MIT",
   "scripts": {
-    "clean": "gatsby clean",
-    "build": "gatsby build",
-    "develop": "gatsby develop",
-    "format": "prettier --write '**/*.js'",
-    "test": "echo \"Error: no test specified\" && exit 1"
+    "dev": "astro dev",
+    "build": "astro build",
+    "preview": "astro preview",
+    "test": "node --test",
+    "format": "prettier --write '**/*.{js,jsx,mjs}'"
+  },
+  "dependencies": {
+    "@astrojs/react": "^7.1.1",
+    "@fontsource-variable/hanken-grotesk": "^5.3.0",
+    "@fontsource-variable/jetbrains-mono": "^5.3.0",
+    "@fontsource-variable/space-grotesk": "^5.3.0",
+    "astro": "^7.4.2",
+    "react": "^19.0.0",
+    "react-dom": "^19.0.0"
   },
   "devDependencies": {
     "prettier": "^3.1.0"
   },
   "engines": {
-    "node": ">=18.0.0"
-  },
-  "resolutions": {
-    "ajv": "^8.12.0",
-    "ajv-keywords": "^5.1.0",
-    "schema-utils": "^4.2.0"
+    "node": ">=22.23.4"
   }
 }