security: implement pentest remediation (RATE-002, CLI-001)

RATE-002: Proactive rate limit cleanup when entries exceed threshold
- Add RATE_LIMIT_CLEANUP_THRESHOLD config (default 0.8)
- Trigger cleanup before hitting hard limit
- Prevents memory exhaustion under sustained load

CLI-001: Validate clipboard tool paths against trusted directories
- Add TRUSTED_CLIPBOARD_DIRS for Unix system paths
- Add TRUSTED_WINDOWS_PATTERNS for Windows validation
- Reject tools in user-writable locations (PATH hijack prevention)
- Use absolute paths in subprocess calls

PENTEST_PLAN.md

@@ -245,8 +245,8 @@ Testing uses specialized Claude subagents for different security domains, with f
 - [x] **BURN-001**: Track HEAD requests as paste access for burn-after-read
 - [x] **BURN-002**: Add test for HEAD-then-GET race condition
 - [x] **RATE-001**: Add maximum entries limit to rate limit storage
-- [ ] **RATE-002**: Add automatic cleanup trigger when threshold exceeded
-- [ ] **CLI-001**: Validate clipboard tool paths against allow-list
+- [x] **RATE-002**: Add automatic cleanup trigger when threshold exceeded
+- [x] **CLI-001**: Validate clipboard tool paths against allow-list
 
 ### Medium-term (Medium)