Security: add explicit opt-in for deprecated plugin runtime exec (#20874)

Merged via /review-pr -> /prepare-pr -> /merge-pr.

Prepared head SHA: de69f81725
Co-authored-by: mbelinky <132747814+mbelinky@users.noreply.github.com>
Co-authored-by: mbelinky <132747814+mbelinky@users.noreply.github.com>
Reviewed-by: @mbelinky

src/config/zod-schema.ts

@@ -623,6 +623,12 @@ export const OpenClawSchema = z
           })
           .strict()
           .optional(),
+        runtime: z
+          .object({
+            allowLegacyExec: z.boolean().optional(),
+          })
+          .strict()
+          .optional(),
         slots: z
           .object({
             memory: z.string().optional(),