refactor(opencode): remove core service layer exports

[jlongster]

Summary

• remove remaining foundational opencode defaultLayer exports
• internalize service implementation layers behind nodes
• leave RuntimeFlags.layer as the explicit override helper exception

Testing

• bun typecheck
• bun turbo typecheck (pre-push)

Stack

1. refactor(opencode): build runtimes from layer nodes #34515 refactor(opencode): build runtimes from layer nodes
2. refactor(opencode): use layer nodes in remaining harnesses #34516 refactor(opencode): use layer nodes in remaining harnesses
3. refactor(opencode): remove app service layer exports #34517 refactor(opencode): remove app service layer exports
4. refactor(opencode): remove core service layer exports #34518 refactor(opencode): remove core service layer exports
5. refactor(opencode): keep plugin pty environment route local #34519 refactor(opencode): keep plugin pty environment route local

Previous: #34517
Base: remove-app-layers
Next: #34519