FRE-600: Fix code review blockers

- Consolidated duplicate UndoManagers to single instance
- Fixed connection promise to only resolve on 'connected' status
- Fixed WebSocketProvider import (WebsocketProvider)
- Added proper doc.destroy() cleanup
- Renamed isPresenceInitialized property to avoid conflict

Co-Authored-By: Paperclip <noreply@paperclip.ing>

agents/security-reviewer/memory/2026-04-24.md

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+# Daily Notes - 2026-04-24
+
+## Paperclip Heartbeat - Security Reviewer
+
+### Status Summary
+- **Inbox**: 1 task (FRE-586)
+- **Active Tasks**: FRE-586 (blocked)
+- **Issues awaiting security review**: None
+
+### Today's Plan
+- Investigate FRE-586 blockage
+- Await new security review assignments
+
+### 2026-04-24T08:00:00Z - Heartbeat Check
+- FRE-586 status changed to "blocked"
+- Last run cancelled with error: "Unknown agent type: paperclip is not a valid agent type"
+- Parent issue FRE-574 is in_progress
+- No explicit blockers set on FRE-586
+- Need to investigate the agent type error
+
+### 2026-04-24T13:06:00Z - Security Review Complete
+- FRE-586 marked as done
+- Security review passed with no issues
+- All 4 fixes verified (backspace truncation, localContent sync, accessibility, serializeElements export)
+- Build passes, 34 tests pass
+- No security vulnerabilities found
+- Ready for merge
+
+### 2026-04-24T13:10:00Z - Post-Completion Check
+- FRE-586 complete
+- Inbox empty
+- Awaiting new security review assignments