feat(backend): add safety framework foundation (Phase A) (#63)

Core safety framework architecture for autonomous agent guardrails:

**Core Components:**
- SafetyGuardian: Main orchestrator for all safety checks
- AuditLogger: Comprehensive audit logging with hash chain tamper detection
- SafetyConfig: Pydantic-based configuration
- Models: Action requests, validation results, policies, checkpoints

**Exception Hierarchy:**
- SafetyError base with context preservation
- Permission, Budget, RateLimit, Loop errors
- Approval workflow errors (Required, Denied, Timeout)
- Rollback, Sandbox, Emergency exceptions

**Safety Policy System:**
- Autonomy level based policies (FULL_CONTROL, MILESTONE, AUTONOMOUS)
- Cost limits, rate limits, permission patterns
- HITL approval requirements per action type
- Configurable loop detection thresholds

**Directory Structure:**
- validation/, costs/, limits/, loops/ - Control subsystems
- permissions/, rollback/, hitl/ - Access and recovery
- content/, sandbox/, emergency/ - Protection systems
- audit/, policies/ - Logging and configuration

Phase A establishes the architecture. Subsystems to be implemented in Phase B-C.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/app/services/safety/rollback/__init__.py

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+"""${dir} module."""