release: prepare v0.3.0

[CoderDeltaLAN]

Summary

Prepare agent-rules-kit v0.3.0 for release.

This PR:

• cuts package metadata from 0.2.3 to 0.3.0;
• updates __version__ to 0.3.0;
• converts the v0.3.0 release notes draft into final release notes;
• moves CHANGELOG [Unreleased] into a dated [0.3.0] section;
• updates README, SECURITY, SUPPORT, OUTPUTS, and roadmap truth from release-candidate to published-release state for the release branch;
• expands installed-package smoke checks for doctor, budget, and explain.

Validation already run locally

• 142 tests OK
• ./scripts/check.sh OK
• ruff OK
• text hygiene OK
• git whitespace OK
• build wheel/sdist OK
• twine check OK
• clean wheel smoke OK
• clean sdist smoke OK
• agent-rules-kit --version returns agent-rules-kit 0.3.0
• check --format json smoke OK
• doctor smoke OK
• budget smoke OK
• explain AIRK-GOV003 smoke OK
• explain --list smoke OK

Release gates still pending after merge

Do not publish from this PR branch.

After PR merge and main CI success by exact SHA, the release phase must still:

• create GitHub Release v0.3.0 from the exact merged main SHA;
• upload wheel, sdist, and SHA256SUMS.txt assets;
• wait for Publish PyPI workflow success;
• verify PyPI JSON 0.3.0;
• run clean install smoke from PyPI agent-rules-kit==0.3.0;
• verify tag, release, PyPI, CI, and cleanup branch.