Add PyPI release workflow with PEP 740 attestations

[negillett]

Summary

• Add release-pypi.yml to build, publish, and sign Python SDK releases on
  SemVer tags via PyPI trusted publishing (PEP 740 attestations) and Cosign
  metadata on GitHub Releases.
• Set PyPI package metadata in pyproject.toml (intentproof, project URLs,
  keywords) per ADR-012 SDK distribution posture.
• Refresh the signing dry-run workflow to use pytest and the current reusable
  signing workflow pin.

Test plan

• PATH=".venv/bin:$PATH" pytest -q (91 passed, 100% coverage)
• PATH=".venv/bin:$PATH" bash ./scripts/check-coverage.sh 95
• After merge: configure PyPI trusted publisher for
  IntentProof/intentproof-sdk-python, tag v0.2.0, verify
  pip download --require-hashes + cosign verify-blob (Task acceptance)

Review

• Internal review found no blocking issues. Residual risk: first real PyPI
  publish requires PyPI trusted-publisher configuration and a tagged release
  run before acceptance can be marked complete.

[cursor]

PR Summary

Medium Risk
Introduces new automated release/publishing and signing workflows plus a PyPI package rename/metadata update; main risk is misconfigured release automation or publishing under the wrong package/version.

Overview
Adds a new release-pypi.yml workflow that runs on vMAJOR.MINOR.PATCH tags (or manual dispatch) to test with pytest+coverage, sync pyproject.toml version from the tag/ref, build sdists/wheels, publish to PyPI using the reusable release-build-sign.yml workflow (with PEP 740 attestations), sign the built artifacts, and upload distributions plus signing metadata to the GitHub Release.

Updates the signing dry-run workflow to use pytest with dev deps and switches the reusable signing workflow pin/subject_name to intentproof.

Adjusts pyproject.toml package metadata for PyPI distribution, including renaming the project to intentproof and adding authors/keywords and project URLs.

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