feat: zero-dependency npm registry setup script

scripts/setup-npm-registry.mjs configures npm to pull @redsentech
packages (e.g. @redsentech/lean-harness) from GitHub Packages:

- Discovers a token: --token flag > env (NPM_REGISTRY_TOKEN/
  GITHUB_TOKEN/GH_TOKEN) > `gh auth token` > interactive masked prompt
  (raw-mode stdin, no echo, no external deps).
- Verifies the token against the GitHub API, reports the authenticated
  login, and warns if the OAuth scopes are missing read:packages/
  write:packages.
- Writes/updates only its own two lines in .npmrc (scope registry +
  auth token), preserving every other line; idempotent on rerun.
- Never prints the full token (masked in all output).
- --dry-run, --unset (clean removal), --local, --scope, --registry,
  --skip-verify, --npmrc <path> flags.
- Confirms the result with `npm whoami --registry ...`.

Verified live end-to-end: real GitHub PAT via `gh auth token` ->
verified against api.github.com -> written to a scratch .npmrc ->
`npm whoami` succeeded. Also confirmed the scope-warning is accurate:
installing @redsentech/lean-harness with a token lacking read:packages
correctly 403s, exactly as the script warns it will.

Adds tests/setup-npm-registry.test.mjs (7 hermetic tests: dry-run,
write+preserve, token never printed, idempotent rerun, --unset,
custom scope/registry, clean failure with no token). Adds
"setup-npm-registry" npm script. Documents the script in README (The
`lh` CLI section) and docs/QUICKSTART.md (install + troubleshooting).

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

package.json

@@ -31,7 +31,8 @@
     "validate": "node scripts/validate.mjs",
     "test": "node --test tests/*.test.mjs",
     "lh": "node src/cli.mjs",
-    "onboard": "node scripts/onboard.mjs"
+    "onboard": "node scripts/onboard.mjs",
+    "setup-npm-registry": "node scripts/setup-npm-registry.mjs"
   },
   "dependencies": {
     "web-tree-sitter": "^0.25.10"