refactor(plugin): remove metadata assertions

Guard optional plugin metadata and pane identifiers before passing them to cleanup and warning paths.

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

src/shared/log-legacy-plugin-startup-warning.ts

@@ -16,7 +16,7 @@ export function logLegacyPluginStartupWarning(deps: LogLegacyPluginStartupWarnin
   const migrateLegacyPluginEntryFn = deps.migrateLegacyPluginEntry ?? migrateLegacyPluginEntry
 
   const result = checkForLegacyPluginEntryFn()
-  if (!result.hasLegacyEntry) {
+  if (!result.hasLegacyEntry || !result.configPath) {
     return
   }
 
@@ -34,7 +34,7 @@ export function logLegacyPluginStartupWarning(deps: LogLegacyPluginStartupWarnin
     + ` Attempting auto-migration...`,
   )
 
-  const migrated = migrateLegacyPluginEntryFn(result.configPath!)
+  const migrated = migrateLegacyPluginEntryFn(result.configPath)
   if (migrated) {
     console.warn(`[oh-my-openagent] Auto-migrated opencode.json: ${result.legacyEntries.join(", ")} -> ${suggestedEntries.join(", ")}`)
   } else {