refactor: Migrate Gulp build to GitHub Actions

Proposal: Remove Gulp from the data build

Gulp currently orchestrates the pipeline that produces src/_data/tools.yaml (gulpfile.js/index.js), piping content through lib/data/transform/* functions via gulp-transform/gulp-rename. Since GitHub Actions already schedules and runs this build, Gulp's task-runner/stream layer adds indirection without adding capability.

Finding: every lib/data/transform/* function is already Gulp-agnostic — each is a plain (content) => string | Promise<string> function, confirmed by their existing unit tests, which call them directly with a single string argument. None of them use Gulp's Vinyl file argument, and all GitHub API auth (GH_API_USERNAME/GH_API_TOKEN/GH_API_CONCURRENCY_LIMIT) is already read from process.env inside the transform functions, not injected by Gulp. So this is purely an orchestration swap — no changes to lib/data/** logic.

Approach:

• Replace gulpfile.js/index.js with scripts/build-data.js, a plain Node script (--task full|metadata) that chains the same lib/data functions in the same order, using the same yargs flags (--metadata, --env-file, --output-dir, --dry-run) and dotenv loading as today.
• Move the non-Gulp-specific config (gulpfile.js/metadata.json, gulpfile.js/.env) to config/metadata.json / config/.env.
• Update package.json's build:data:full/build:data:metadata scripts to call the new script; drop the gulp, gulp-cli, gulp-rename, gulp-transform dependencies.
• No GitHub Actions workflow changes needed — full.yaml/metadata.yaml invoke the build only via yarn run build:data:full/build:data:metadata, so preserving those script names means the workflows keep working unchanged.
• Verify equivalence once during PR review by diffing old-Gulp vs new-script output (tools.yaml, raw-sources.yaml) under --dry-run; no new automated test needed since the underlying lib/data/transform/* logic (and its existing test coverage) is untouched.
• Update README.md, CONTRIBUTING.md, and CLAUDE.md references to Gulp accordingly.

Single PR, low risk — mechanical orchestration swap only.

Start with gulpfile.js/index.js, the existing lib/data/transform/* unit tests, and the build:data:full and build:data:metadata scripts in package.json. Trace the current task order and flags before creating scripts/build-data.js and moving the configuration files. Done means the scripts produce equivalent tools.yaml and raw-sources.yaml output under --dry-run, Gulp dependencies and references are removed, and the existing GitHub Actions commands still work.