docs: add contributor safety and governance training

[ibloud]

Purpose

Add a reusable professional-safety orientation to the contributor on-ramp while preserving the boundary between training, canonical rules, and private case evidence.

Changes

• add CONTRIBUTOR_SAFETY_AND_GOVERNANCE.md
• add the safety orientation to Phase 0 and the candidate checklist
• teach collaborator vetting, written scope, consent boundaries, staged access, revocation, termination clauses, and kill fees
• link trainees to the engine variant workflow, template, and Dread Pressure example
• clarify that training does not grant employment, Playtester status, canon authority, repository ownership, or production access
• clarify that private allegations, explicit material, residential details, and identifiable evidence do not belong in the training repository

Boundaries

• no canonical rules or exercise behavior changed
• no identifiable Violet case chronology or private evidence included
• no Patreon, Buy Me a Coffee, Discord, or commerce operations changed
• engine PR #38 remains the variant-governance authority under review

Verification

• branch is four commits ahead of main and zero behind
• all four intended files read back successfully
• only documentation changed; TypeScript source and mandatory tests are untouched