Replace the legacy toast regex writer with the shared atomic helper and route legacy plugin call sites through small wrappers so existing mock.module tests stop leaking across the suite.
Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)
Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>
- 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
YeonGyu-Kim