Confirm and add explicit repository license

## Summary

`a2aproject/a2a-itk` does not currently have a detected repository license.

Observed from the public repo:

- GitHub `licenseInfo`: `null`
- Root contents do not include `LICENSE`
- `README.md` does not appear to include a license section

Sibling repo comparison:

- `a2aproject/a2a-tck` has an Apache-2.0 `LICENSE` file and GitHub detects `Apache License 2.0`.

## Why this matters

The ITK is meant to be reused by SDK repositories and CI/nightly compatibility workflows. An explicit license helps downstream SDK maintainers and enterprise users understand whether they can copy, vendor, run, or adapt the integration kit.

## Suggested maintainer-owned decision

Could maintainers confirm the intended license for `a2a-itk`?

If the intended license is Apache-2.0 to match the other A2A repositories, I can follow up with a small PR that only adds:

- root `LICENSE`
- a short `## License` section in `README.md`

I am not opening that PR now because license choice should be explicit from maintainers.

Check the sibling repository a2aproject/a2a-tck for its LICENSE file and README license section. If the maintainers confirm Apache-2.0, add a LICENSE file with the same content to the root of this repo and a '## License' section in README.md. Verify the changes by ensuring GitHub detects the license.

索引モデルが issue の本文から書いたものです。