fix(sandbox): sanitize @ in container names for WhatsApp LID chat IDs (#1031)

WhatsApp LID-format chat IDs contain @ (e.g. 551152861098:5@s.whatsapp.net)
which is invalid in Docker container names, causing sandbox creation to fail
for any WhatsApp-triggered agent session.

Add @ to the sanitizeKey replacer alongside the existing : / . and space
characters. Adds a test case with a realistic WhatsApp LID key.

Fixes #1029

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

internal/sandbox/docker.go

@@ -454,6 +454,7 @@ func sanitizeKey(key string) string {
 		"/", "-",
 		" ", "-",
 		".", "-",
+		"@", "-",
 	).Replace(key)
 
 	if len(safe) > 50 {

internal/sandbox/docker_test.go

@@ -97,6 +97,7 @@ func TestSanitizeKey(t *testing.T) {
 		{"simple", "simple"},
 		{"has/slash", "has-slash"},
 		{"has space", "has-space"},
+		{"agent:chloe:whatsapp:551152861098:5@s.whatsapp.net", "agent-chloe-whatsapp-551152861098-5-s-whatsapp-net"},
 		{strings.Repeat("x", 100), strings.Repeat("x", 50)},
 	}
 	for _, tc := range tests {