Rename sandbox prefix to cl-, add MMDS metadata, fix proxy port routing

- Change sandbox ID prefix from sb- to cl- (capsule) throughout
- Fix proxy URL regex character class: base36 uses 0-9a-z, not just hex
- Add MMDS V2 config and metadata to VM boot flow so envd can read
  WRENN_SANDBOX_ID and WRENN_TEMPLATE_ID from inside the guest
- Pass TemplateID through VMConfig into both fresh and snapshot boot paths

internal/validate/name_test.go

@@ -11,7 +11,7 @@ func TestSafeName(t *testing.T) {
 		{"simple", "minimal", false},
 		{"with-dash", "template-abc123", false},
 		{"with-dot", "my-snapshot.v2", false},
-		{"sandbox-id", "sb-12345678", false},
+		{"sandbox-id", "cl-12345678", false},
 		{"single-char", "a", false},
 		{"numbers", "123", false},
 		{"max-length", "abcdefghijklmnopqrstuvwxyz0123456789abcdefghijklmnopqrstuvwxyz01", false},