Move the canonical repo to Gitea; GitHub becomes a private archive

git.karti.ai/PIG/PIG-Demo is now the source of truth, public and anonymously
cloneable. Every self-link in the site and the docs repoints there — Gitea
serves file paths at /src/branch/main/, not /blob/main/, so those needed
rewriting rather than a hostname swap.

CI moves with it. An archived GitHub repo is read-only and its Actions stop
firing, so leaving the workflow there would have meant a repo whose gates
silently never run. .github/ is deleted rather than kept for reference: a
workflow that can never execute is worse than no workflow, because it looks
like coverage.

The Gitea workflow is not a copy. That runner is aarch64 and installs pnpm
through corepack from `packageManager` rather than pnpm/action-setup, uses
checkout@v4 and setup-node@v4, and fetches uv from astral.sh directly. It is
also configured `container.network: host` — nothing here needs a service
container, but the comment says so, because that setting cost the sibling repo
three failed runs.

The header and footer icon changed from the GitHub mark to a neutral one. A
GitHub logo pointing at a Gitea instance is a small lie about where the code
lives, on a site whose argument is that you can go and check it.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_019mt6sHQHEnEYrJZvoMCJSB

src/router.tsx

@@ -111,7 +111,7 @@ function describeError(error: unknown): { heading: string; body: string; detail?
   if (error instanceof Error) {
     return {
       heading: 'This page failed to load',
-      body: 'The rest of the site still works. If this keeps happening, the source is on GitHub and the issue is reproducible from it.',
+      body: 'The rest of the site still works. If this keeps happening, the source is public and the issue is reproducible from it.',
       detail: error.message,
     };
   }