fix: guard against undefined snapshot.skills in applySkillEnvOverridesFromSnapshot

snapshot.skills can be undefined when a session is reset during an API
rate-limit cooldown, causing a TypeError crash that compounds the
original rate-limit issue.

Use nullish coalescing (snapshot.skills ?? []) to safely skip iteration
when skills is not populated.

src/agents/skills/env-overrides.ts

@@ -71,7 +71,7 @@ export function applySkillEnvOverridesFromSnapshot(params: {
   }
   const updates: EnvUpdate[] = [];
 
-  for (const skill of snapshot.skills) {
+  for (const skill of snapshot.skills ?? []) {
     const skillConfig = resolveSkillConfig(config, skill.name);
     if (!skillConfig) {
       continue;