feat(backend): add Phase B safety subsystems (#63)

Implements core control subsystems for the safety framework:

**Action Validation (validation/validator.py):**
- Rule-based validation engine with priority ordering
- Allow/deny/require-approval rule types
- Pattern matching for tools and resources
- Validation result caching with LRU eviction
- Emergency bypass capability with audit

**Permission System (permissions/manager.py):**
- Per-agent permission grants on resources
- Resource pattern matching (wildcards)
- Temporary permissions with expiration
- Permission inheritance hierarchy
- Default deny with configurable defaults

**Cost Control (costs/controller.py):**
- Per-session and per-day budget tracking
- Token and USD cost limits
- Warning alerts at configurable thresholds
- Budget rollover and reset policies
- Real-time usage tracking

**Rate Limiting (limits/limiter.py):**
- Sliding window rate limiter
- Per-action, per-LLM-call, per-file-op limits
- Burst allowance with recovery
- Configurable limits per operation type

**Loop Detection (loops/detector.py):**
- Exact repetition detection (same action+args)
- Semantic repetition (similar actions)
- Oscillation pattern detection (A→B→A→B)
- Per-agent action history tracking
- Loop breaking suggestions

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

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

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

@@ -1 +1,15 @@
-"""${dir} module."""
+"""
+Rate Limiting Module
+
+Sliding window rate limiting for agent operations.
+"""
+
+from .limiter import (
+    RateLimiter,
+    SlidingWindowCounter,
+)
+
+__all__ = [
+    "RateLimiter",
+    "SlidingWindowCounter",
+]