fix: remove build artifacts from repo, build SPA in Docker

- Remove web/dist/ from git tracking (build output)
- Add web/dist/ to .gitignore
- Add Node.js web-builder stage to Dockerfile to compile SPA at build time
- Update REPO_POLICIES.md from upstream sneak/prompts (build artifacts policy)

Dockerfile

@@ -1,3 +1,13 @@
+# Web build stage — compile SPA from source
+# node:22-alpine, 2026-03-09
+FROM node@sha256:8094c002d08262dba12645a3b4a15cd6cd627d30bc782f53229a2ec13ee22a00 AS web-builder
+WORKDIR /web
+COPY web/package.json web/package-lock.json ./
+RUN npm ci
+COPY web/src/ src/
+COPY web/build.sh build.sh
+RUN sh build.sh
+
 # Lint stage — fast feedback on formatting and lint issues
 # golangci/golangci-lint:v2.1.6, 2026-03-02
 FROM golangci/golangci-lint@sha256:568ee1c1c53493575fa9494e280e579ac9ca865787bafe4df3023ae59ecf299b AS lint
@@ -5,6 +15,7 @@ WORKDIR /src
 COPY go.mod go.sum ./
 RUN go mod download
 COPY . .
+COPY --from=web-builder /web/dist/ web/dist/
 RUN make fmt-check
 RUN make lint
 
@@ -21,6 +32,7 @@ COPY go.mod go.sum ./
 RUN go mod download
 
 COPY . .
+COPY --from=web-builder /web/dist/ web/dist/
 
 RUN make test