release: cA2A 0.2.0 with Agent Manifest identity binding

[imran-siddique]

Summary

• consume Agent Manifest v0.1 JSON and v0.2 COSE manifests with fail-closed startup identity binding
• promote cA2A from the initial prerelease to the 0.2.0 Developer Preview
• update the roadmap, adoption docs, examples, package maturity, and release notes to match shipped capabilities
• add regression gates preventing prerelease install guidance or Alpha metadata from returning to current adoption docs

Evidence

• full unit + conformance suite: 495 passed, 2 skipped
• release + conformance gates after release edits: 50 passed
• mypy: 43 source files clean
• Ruff lint and format: clean
• Bandit: no findings
• release workflow verifies tag/version equality, wheel and sdist installs, and publishes through trusted publishing

Release

After merge, publish GitHub release �0.2.0. That event triggers the gated PyPI and container release workflows.