♻️ extend async context blocking mechanisms and add comprehensive tests

[kammce]

This commit significantly expands the async context API to support multiple blocking states and adds extensive test coverage. Key changes include:

• Add new blocking methods: block_by_sync() with context pointer, and block_by_external() to support different synchronization patterns
• Make blocking methods return std::suspend_always for improved coroutine suspension ergonomics
• Add context_token class for safe context capture and unblocking operations
• Enhance context state queries with safer std::holds_alternative checks
• Add git-town configuration for workflow management
• Add comprehensive test suite demonstrating resource synchronization patterns between coroutines, including mutex-like behavior and I/O blocking scenarios
• Refactor test infrastructure with reusable test_scheduler implementation.

These enhancements enable more sophisticated coroutine coordination patterns while maintaining the lightweight design of the async context system.