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>

src/pages/404.astro

@@ -0,0 +1,17 @@
+---
+import Base from '../layouts/Base.astro'
+---
+
+<Base title="Page not found · Dennis Thiessen" description="This page does not exist." noindex>
+  <div class="wrap notfound">
+    <p class="notfound__cmd">
+      <span class="chevron" aria-hidden="true"></span>
+      open this-page
+    </p>
+    <h1>No such page.</h1>
+    <p>
+      There is nothing at this address. The terminal on the <a href="/">home page</a> can answer
+      questions about Dennis’s work instead.
+    </p>
+  </div>
+</Base>