Boron Context v0.8.0: rehearse macOS lifecycle

[kforris]

Summary

• add non-overwriting PostgreSQL backup, empty-target restore, and data-preserving uninstall receipts
• add isolated macOS + Codex clean-install, upgrade, restore, rollback, and uninstall rehearsal
• add formal RC checklist, support/security/retention boundaries, and v0.8.0 version alignment
• keep changelog/checklist retrieval noise behind requested roadmap/privacy sources without lowering the frozen evaluation baseline

Verification

• npm run check: 84 passed, 13 environment-skipped; lifecycle unit tests 7/7
• isolated PostgreSQL: migrations 12/12, integration 13/13, idempotent rerun
• swift test 5/5 and release build
• npm audit --omit=dev --audit-level=high: 0 vulnerabilities
• package dry-run: 0.8.0, 204 entries
• isolated lifecycle: 29/29 steps, 62.697s, 0.7.8 -> 0.8.0 -> 0.7.8 rollback, production DB untouched
• production-shaped eval: recall@5 0.888889, MRR 0.768519 (frozen 0.736111), source coverage 0.888889, wrong-project retrieval 0

Public release, Marketplace submission, signing, payment, and marketing remain outside this PR.