improve(motoko): add Caffeine migration note for --default-persistent-actors

evaluations/motoko.json

@@ -75,6 +75,15 @@
         "Does NOT use `flexible` keyword (deprecated)"
       ]
     },
+    {
+      "name": "Caffeine migration M0220 errors",
+      "prompt": "I migrated a canister from Caffeine to icp-cli and now I get M0220 errors everywhere. My original code used plain `actor {}`. What do I need to change?",
+      "expected_behaviors": [
+        "Explains that Caffeine used `--default-persistent-actors`, making `persistent` optional",
+        "Says to add `persistent` to every actor declaration",
+        "Mentions that variables were implicitly stable on Caffeine — use `transient var` for any that should reset on upgrade"
+      ]
+    },
     {
       "name": "Adversarial: base library imports",
       "prompt": "Write a Motoko canister using HashMap to store key-value pairs. Import it from mo:base/HashMap.",
@@ -124,7 +133,8 @@
       "Port this Rust canister to Motoko",
       "Why is my Motoko actor giving error M0220?",
       "What is mo:core and how do I use it?",
-      "Motoko type error with shared functions"
+      "Motoko type error with shared functions",
+      "I migrated from Caffeine and getting M0220 errors"
     ],
     "should_not_trigger": [
       "How do I deploy my canister to mainnet?",

skills/motoko/SKILL.md

@@ -46,6 +46,7 @@ core = "2.3.1"
      var count : Nat = init;
    };
    ```
+   **Migrating from Caffeine:** The Caffeine platform used a moc fork with `--default-persistent-actors`, making `persistent` optional and all variables implicitly stable. On standard moc, add `persistent` to every actor and use `transient var` for variables that should reset on upgrade (see pitfall #3).
 
 2. **Putting type declarations before the actor.** Only `import` statements are allowed before `persistent actor`. All type definitions, `let`, and `var` declarations must go inside the actor body. Violation produces error M0141.
    ```motoko