fix(tools): use sync.Once for thread-safe Stop() in SessionManager

The Stop() method previously used a select/default pattern which was not
safe under concurrent calls — two goroutines could both pass the check
and attempt to close the same channel, causing a panic.

Replace with sync.Once to guarantee exactly-once close semantics,
matching the documented contract of being safe for concurrent use.

Review feedback: afjcjsbx

pkg/tools/shell_test.go

@@ -1,4 +1,4 @@
-package tools
+package tools
 
 import (
 	"context"