Remove three unused methods/parameter identified by static analysis:
- canRequeue(): never integrated into scheduling flow
- runMetricsClient clientID param: accepted but never used
- getUsageLocked(): callers inline the logic
Fixes IDE warnings about unused code per AGENTS.md cleanup discipline.
- Add plugin_quota.go with GPU quota management for scheduler
- Update scheduler hub and protocol for plugin support
- Add comprehensive plugin quota unit tests
- Update gang service and WebSocket queue integration tests