fix(effect): preserve logger context in prompt runs

[kitlangton]

Summary

• merge Observability.layer directly into AppRuntime so it installs the same logger set as makeRuntime(...)
• switch the internal SessionPrompt promise/fork bridge to Effect.runPromiseWith(ctx) and Effect.runForkWith(ctx) so prompt runs keep the active logger context
• add a regression test proving AppRuntime includes EffectLogger just like makeRuntime(...)

Testing

• bun run test test/effect/app-runtime-logger.test.ts test/session/prompt-effect.test.ts
• bun run typecheck

Notes

• replacement for the stale fix(effect): preserve logger context in prompt runs #22486 branch