release: cut v0.2.1 metadata

[CoderDeltaLAN]

Summary:

• Cuts v0.2.1 metadata from the post-v0.2.0 unreleased changes.
• Updates package version metadata, package version, changelog, README installation text, and project release URL.
• Makes the CLI version test follow version instead of hardcoding a release string.

Verification:

• PYTHONPATH=src python -m agent_rules_kit.cli --version returned agent-rules-kit 0.2.1.
• Full local check passed: 118 tests, Ruff, text hygiene, git whitespace.
• Build smoke passed for wheel and sdist.
• Wheel and sdist install smoke checks passed from clean temporary environments.

Scope:

• CHANGELOG.md
• README.md
• pyproject.toml
• src/agent_rules_kit/init.py
• tests/test_cli.py

Not included:

• No tag creation.
• No GitHub Release publication.
• No release asset upload.