feat(bmm): add bmad-investigate skill

Forensic case investigation under Amelia's menu (IN). Evidence-graded
findings (Confirmed / Deduced / Hypothesized), hypothesis discipline,
structured case-file artifact. Single procedure that calibrates between
defect-chasing and area-exploration based on the input.

Wires bmad-create-prd discovery to pick up case files as PRD input.
Public explainer doc, workflow-map Phase 4 row, EN + FR.