fix(bmm): unwrap case-file template, tighten PRD discovery glob

Two PR review fixes:

- Strip the surrounding markdown code fence from case-file-template.md
  so initializing a case file doesn't nest the whole artifact inside a
  code block. The template is now the structure directly, with usage
  notes in an HTML comment that doesn't render. Matches the BMM idiom
  used by bmad-create-story/template.md.
- Drop the stale `*-archaeology.md` reference from bmad-create-prd
  step-01 discovery. The single-procedure restructure removed
  archaeology as a distinct mode, so case files only carry the
  `*-investigation.md` suffix. Tighten the glob accordingly.