Golang Concurrency

This is a comprehensive concurrency engineering skill that treats every goroutine as a liability until proven necessary. It covers three modes: writing new concurrent code with channels and sync primitives, reviewing PRs for leaks and race conditions, and auditing existing codebases with up to 5 parallel sub-agents. The checklist is practical (how will it exit, who owns the channel, can I wait for it) and the anti-patterns are specific: time.After in loops, missing ctx.Done in select, closing from the receiver side. It includes a solid primitives reference covering WaitGroup vs errgroup, channels vs mutexes, and when to reach for singleflight or sync.Map. If you're writing Go services that spawn goroutines, this gives you a structured approach to not leaking them.