Initial commit: Go SRE interview exercises

Includes solved 02-worker-pool and 09-safe-counter (mutex, atomic, sharded).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01GbnJj9ZqEx9nUvzkjdwfMq

10-gotchas/ex09-waitgroup/main.go

@@ -0,0 +1,22 @@
+// PREDICT: what are the possible outputs? What's the race here even if it
+// "usually works"? Run with -race a few times.
+package main
+
+import (
+	"fmt"
+	"sync"
+)
+
+func main() {
+	var wg sync.WaitGroup
+	count := 0
+	for i := 0; i < 3; i++ {
+		go func() {
+			wg.Add(1) // <-- what's wrong with Add here?
+			defer wg.Done()
+			count++
+		}()
+	}
+	wg.Wait()
+	fmt.Println("count:", count)
+}