refactor: move subprocess worker to workers/ directory

[Blackgaurd]

What changed

Moved runtime/isolated_worker.py → workers/subprocess_worker.py and the corresponding test file from tests/runtime/ → tests/workers/.

Updated runtime/runner.py to point WORKER_PATH at the new location.

Why

isolated_worker.py lived in runtime/ alongside server-side infrastructure (config loading, registry, venv management), but it's fundamentally different: it's a standalone subprocess entry point that runs in a separate Python interpreter, never imported by the server. A dedicated workers/ directory makes this distinction explicit.

Benefit

Clearer separation of concerns in the directory structure. runtime/ is now purely server-side infrastructure; workers/ holds subprocess entry points.

[github-actions]

• refactor: move subprocess worker to workers/ directory 👈
  • docs: document subprocess_worker as single blocking worker #341
    • docs: update references for subprocess_worker rename #342