fix: [ENG-2860] address PR #682 review feedback

- constants: add _index.html to CONTEXT_TREE_GITIGNORE_PATTERNS — the
  index is a derived artifact and must be kept out of user git, same
  as _index.md (PR claimed parity; the entry was missing).
- validate-html-index: the <bv-index> root check now inspects document
  children, not "anywhere in the tree". A nested <bv-index> inside a
  <bv-index-domain> previously satisfied the guard via walkElements'
  depth-first scan — the validator is the FE contract, so the root
  check must be a true root check. Adds a regression test.
- schemas: tighten `generatedat` from a bare non-empty string to
  z.string().datetime() — the generator always emits an ISO 8601
  instant; pinning it makes the contract explicit for the FE viewer.