chore(scanner): remove orphan src/index.js (#25)

## Summary
- Removes `tools/stale/packages/scanner/src/index.js`, an orphan from
the TS era.
- After the rescript reconcile (`9d663eb`) `src/Index.res` →
`src/Index.mjs` became the package main (per `package.json`), so the
lowercase file is now unreferenced — `grep -r "scanner/src/index"`
across the workspace returns nothing.

## Test plan
- [x] No importer in workspace (`grep` clean).
- [x] `rescript build` in `tools/stale/packages/scanner` still passes.
- [ ] Reviewer: confirm `npm run build --workspaces` still resolves the
scanner entry via `Index.mjs`.

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Co-authored-by: Claude Opus 4.7 <noreply@anthropic.com>

Author: hyperpolymath