Update documentation to reflect the new concurrent test execution
capabilities and add guidance on run ID isolation.
AGENTS.md:
- Add examples for running multiple tests concurrently
- Document run ID format and container naming conventions
- Update "Critical Notes" to explain isolation mechanisms
.claude/agents/headscale-integration-tester.md:
- Add "Concurrent Execution and Run ID Isolation" section
- Document forbidden and safe operations for cleanup
- Add "Agent Session Isolation Rules" for multi-agent environments
- Add 6th core responsibility about concurrent execution awareness
- Add ISOLATION PRINCIPLE to critical principles
- Update pre-test cleanup documentation
