fix(02): revise plans based on checker feedback

- 02-01 Task 3: Add concrete REPL verification command for PluginManifest.parse
- 02-02 Task 1: Add FieldDef case class to placeholder types in PluginRegistration.scala
- 02-02 Task 1: Document PluginEnv as placeholder to be expanded in 02-03
- 02-03 Task 3: Remove duplicate Main.scala code, keep only corrected version
- 02-03: Add SummerPlugin.scala to files_modified for PluginEnv update

.planning/phases/02-plugin-system/02-01-PLAN.md

@@ -282,7 +282,25 @@ dependencies:
   golem15.user: "^1.0.0"
 ```
 
-Then verify parsing works by checking the code compiles and the types are correct.
+Then test discovery works by running this command in the Mill REPL:
+```bash
+./mill -i summercms.console
+```
+
+Once in the REPL, verify parsing:
+```scala
+import plugin._
+val yaml = """
+vendor: test
+name: sample
+version: 1.0.0
+description: Test plugin
+"""
+val result = PluginManifest.parse(yaml)
+println(result) // Should print Right(PluginManifest(test,sample,1.0.0,...))
+```
+
+Exit REPL with `:quit` after verification.
   </verify>
   <done>PluginManifest parses YAML manifests, PluginDiscovery scans plugins/ directory, plugins/ directory exists</done>
 </task>