build(layer2+3): propagate .build/+dist/ to Docker/Electron/CI; codify light deploy; docs

Layer 3: Dockerfile COPY .next/standalone -> .build/next/standalone (+cache mount);
electron stage -> .build/electron-standalone + extraResources; CI asserts dist/server.js;
eslint ignores .build/**+dist/**. Layer 2: deploy-vps-* skills use build:release + rsync(dist)
-> remote app/ + pm2 restart + BUILD_SHA verify (drop npm-i-g/legacy-peer-deps/manual-wreq).
Docs (RELEASE_CHECKLIST/CONTRIBUTING/AGENTS/CHANGELOG) describe src/+.build/+dist/ layout.
Verified: docker build exit 0 + health 200; electron stage OK; no stale build-output refs.

.dockerignore

@@ -9,6 +9,7 @@
 # Dependencies and build output
 node_modules
 .next
+.build
 out
 build
 dist