[codex] clarify actions-work-items README

[joshyorko]

Summary

• Reworks work-items/README.md so the PyPI page leads with the work item model.
• Adds Robocorp-style examples for inputs.current, iterating inputs, payloads, files, failures, adapters, and migration.
• Moves backend and Action Server details after the core usage flow.

Closes #69.

Validation

• uv run --with build --with twine -- python -m build
• uv run --with twine -- python -m twine check dist/*

[joshyorko]

Updated after comparing against robocorp/robocorp workitems source, not just the rendered docs.

Key corrections in the PR:

• actions-work-items does not auto-reserve an initial input like Robocorp's task-cache/Control Room path; the README now uses inputs.reserve() before inputs.current.
• Added compatibility notes for outputs.create(files=...), get_file(), get_email(), and State.DONE vs Robocorp's COMPLETED value.
• Kept the Robocorp-style queue model and import migration, but avoided implying a byte-for-byte clone of the Control Room library.

Validation after the update:

• PYTHONPATH=src uv run --with pytest --with pytest-asyncio --env-file /dev/null --no-project -- python -m pytest tests/work_items_tests/test_workitems_compat_api.py tests/work_items_tests/test_adapter_factory.py
• uv run --with build --with twine -- python -m build
• uv run --with twine -- python -m twine check dist/*