feat(execution): add plugin execution observers

[aryasaatvik]

Summary

• adds an Effect-native execution observer event contract in @executor-js/sdk
• threads observer emission through the core execution engine for execution, tool, and interaction lifecycle events
• lets plugins provide runtime execution observers, with local/cloud hosts composing plugin observers into the engine
• covers observer composition and engine emission with targeted tests

Stack Notes

This PR is the base observer primitive. The fork cannot represent the dependent branches as stacked PRs, so the follow-up work is pushed as fork branches only:

• History plugin diff: aryasaatvik/executor@feat/execution-observers...feat/execution-history-plugin
• Metrics/observability diff: aryasaatvik/executor@feat/execution-observers...feat/execution-metrics-observability

No PRs are opened for those follow-up branches yet.

Validation

• packages/core/sdk: bun run test src/execution-observer.test.ts
• packages/core/execution: bun run test src/engine.test.ts
• packages/core/sdk: bun run typecheck
• packages/core/execution: bun run typecheck
• apps/local: bun run typecheck
• apps/cloud: bun run typecheck
• root: bun run lint
• root: bun run format:check
• root: git diff --check