ci(nsite): build a downloadable site archive on every push

Add .github/workflows/nsite-archive.yml (push to any branch +
workflow_dispatch): npm ci + npm run build, then upload dist/ as an
artifact (bitvid-site-<branch>-<shortsha>, 30d retention). GitHub serves
it as a .zip with index.html at the root, ready for a manual nsite
publish. The workflow holds no signing key and publishes nothing.

Also add an `archive` npm script for the same output locally
(bitvid-site.zip, now gitignored), and land docs/nsite-mirror-plan.md
revised to the manual-publish + auto-archive model: sign client-side via
NIP-07 or local nsyte, never an nsec/nbunksec in CI. The prior CI-signing
workflow is retained as a labeled Phase-3 reference option.

Co-Authored-By: Claude Opus 4.8 <[email protected]>
Claude-Session: https://claude.ai/code/session_01DwbE3uTiFaKRNjx5vP2muV

package.json

@@ -11,6 +11,7 @@
     "prepare": "npm run build:css",
     "serve": "npx serve",
     "build": "node scripts/build-dist.mjs && npm run verify:dist:deploy-artifact",
+    "archive": "npm run build && rm -f bitvid-site.zip && (cd dist && zip -qr ../bitvid-site.zip .) && echo 'Wrote bitvid-site.zip (upload to your nsite deployer).'",
     "build:css": "postcss css/tailwind.source.css -o css/tailwind.generated.css",
     "build:beacon:bundle": "node scripts/build-beacon.mjs",
     "build:beacon": "npm run build:beacon:bundle && npm run lint:inline-styles",