fix(#2823): auto-migrate legacy plugin name and warn users at startup

- logLegacyPluginStartupWarning now emits console.warn (visible to user,
  not just log file) when oh-my-opencode is detected in opencode.json
- Auto-migrates opencode.json plugin entry from oh-my-opencode to
  oh-my-openagent (with backup)
- plugin-config.ts: add console.warn when loading legacy config filename
- test: 10 tests covering migration, console output, edge cases

src/config/schema/hooks.ts

@@ -52,6 +52,7 @@ export const HookNameSchema = z.enum([
   "read-image-resizer",
   "todo-description-override",
   "webfetch-redirect-guard",
+  "legacy-plugin-toast",
 ])
 
 export type HookName = z.infer<typeof HookNameSchema>