Add audit.toml for cargo-audit quality gate configuration.
Per Phase 0.4 Quality Targets, the cargo-audit gate enforces:
- Warnings denied (--deny warnings)
- >= medium severity advisories block PR merge
- Unmaintained advisories ignored (informational only)
The audit.toml file provides an allow-list format for intentionally
ignored advisories, each requiring a justification note.
Co-Authored-By: Claude Code <noreply@anthropic.com>
