fix(tools/skill): invalidate skill cache at session boundary

src/tools/skill/session-skill-cache.ts

@@ -0,0 +1,10 @@
+const seenSessionIDs = new Set<string>()
+
+export function shouldInvalidateSkillCacheForSession(sessionID?: string): boolean {
+  if (!sessionID || seenSessionIDs.has(sessionID)) {
+    return false
+  }
+
+  seenSessionIDs.add(sessionID)
+  return true
+}