docs: document PyPI installation for v0.0.1

[ejames-dev]

Summary

watchpost-cli 0.0.1 is now live on PyPI, so the README no longer needs to say "until registry publication is confirmed, use the source checkout". This PR:

• removes that sentence.
• adds an Install from PyPI section that pins watchpost-cli==0.0.1 and uses a virtual environment.
• notes that the package doesn't include the synthetic example files, so the demos still need a source checkout.

Documentation only.

Verification

• PyPI publication checks:
  • The Publish to pypi from v0.0.1 run built and uploaded from 57656e2.
  • The PyPI wheel and sdist hashes match that run's SHA256SUMS.
  • In a clean venv, pip check, the metadata, and the CLI checks all passed.
  • The report is byte-identical to examples/report.md with network disabled.
  • An existing report file is refused, and the timeout example creates no report.
• I ran the new README commands as written in an empty directory, with uv's Python 3.11 standing in for python3. watchpost --help works.
• Ruff format and lint are clean, and the unittest suite passes.