fix: decouple lint stage from web-builder in Dockerfile

Remove COPY --from=web-builder from the lint stage so it can run
independently and fail fast. Create placeholder files for the
go:embed directive instead. The build stage still uses the real
SPA assets from web-builder.

Update README to reflect that lint no longer depends on web-builder.

README.md

@@ -1854,9 +1854,11 @@ The Dockerfile is a four-stage build:
 1. **web-builder**: Installs Node dependencies and compiles the SPA (JSX →
    bundled JS via esbuild) into `web/dist/`
 2. **lint**: Runs formatting checks and golangci-lint against the Go source
-   (with the built SPA assets copied in)
+   (uses empty placeholder files for `web/dist/` so it runs independently of
+   web-builder for fast feedback)
 3. **builder**: Runs tests and compiles static `neoircd` and `neoirc-cli`
-   binaries (CLI built to verify compilation, not included in final image)
+   binaries with the real SPA assets from web-builder (CLI built to verify
+   compilation, not included in final image)
 4. **final**: Minimal Alpine image with only the `neoircd` binary
 
 ### Binary