refactor: remove script-runner.ts and observability.ts from main src

Follow-up to previous refactor commit: stage the file deletions that
were inadvertently left unstaged.

- packages/core/src/script-runner.ts deleted (moved to planned future subpath)
- packages/core/src/observability.ts deleted (moved to testing/observability.ts)
- packages/core/tests/script-runner.test.ts deleted

Made-with: Cursor

Author: jonzarecki