Allow report_ui builds with explicit input/output paths

[digitalWestie]

Summary

• Make src/report_ui invocable with explicit input/output paths (e.g. --bridging_scores, --summary, --output / --outputDir) so pipelines can build reports without copying artefacts into input/.
• Preserve the existing copy-into-input/ workflow, default output layout (output/static, output/inline), dev / preview / github tasks, and the zero-install npx-based build steps.
• Document the optional CLI flags additively in src/report_ui/README.md and briefly in the root README Step 6.

Test plan

• Legacy: copy fixtures into input/, run npm run inline → output/inline/index.html
• Explicit: node build.js inline --bridging_scores … --summary … --output …/new_report_build.html
• Confirmed legacy and explicit inline HTML are byte-identical on the same fixtures
• npm run static still writes output/static/index.html + quotes.json
• Explicit-path build does not wipe the default output/ tree
• Optional: smoke npm run dev / npm run preview locally

[digitalWestie]

I've tested this manually confirming we get the same outputs using the input folder and the flags method.