refactor(opencode): remove default layer exports

[jlongster]

Summary

• remove opencode service defaultLayer exports and internalize service implementation layer values
• build production runtimes and remaining harnesses through AppNodeBuilder / LayerNode graphs
• keep RuntimeFlags.layer(...) as the explicit test override exception

Testing

• bun typecheck
• bun turbo typecheck (pre-push hook)
• bun test test/server/session-list.test.ts test/server/httpapi-ui.test.ts
• bun test test/effect/runtime-flags.test.ts test/config/tui.test.ts
• bun test test/session/structured-output-integration.test.ts
• bun test test/server/httpapi-session.test.ts
• bun test test/server/httpapi-workspace.test.ts test/server/httpapi-workspace-routing.test.ts test/server/httpapi-instance-context.test.ts test/server/httpapi-promptasync-context.test.ts
• bun test test/server/httpapi-sdk.test.ts

[jlongster]

Superseded by the smaller stacked PRs #34515, #34516, #34517, #34518, and #34519.