onlykey-agent: add PyPI project description and bump to 1.1.16

[0c-coder]

onlykey-agent: add PyPI project description and bump to 1.1.16

The PyPI page for onlykey-agent shows "The author of this package has not
provided a project description", which trips security scanners that block
dependencies with no description.

Changes (agents/onlykey/)

• setup.py: add long_description (from README.md, markdown) and
  long_description_content_type, plus project_urls. Bump version
  1.1.13 → 1.1.16 (1.1.14 and 1.1.15 are already published to PyPI, so
  the next upload must be 1.1.16).
• README.md: new — package description rendered on PyPI (install, usage,
  links).
• MANIFEST.in: new — ship README.md in the sdist so long_description
  resolves when building from the source distribution.

Verification

• python -m build / setup.py sdist succeeds; twine check PASSES.
• Installed the sdist in a clean venv: the three console scripts
  (onlykey-agent, onlykey-gpg, onlykey-gpg-agent) are created and the
  metadata carries the markdown description.

No functional/runtime code changes — packaging metadata only.

[0c-coder]

Companion release: trustcrypto/python-onlykey#90 (onlykey 1.2.11). Packaging-only change here; no dependency, but ships alongside the PQ age-plugin work.