claims-check 0.3.0: reusable release-gate and latest-tag workflows

Added reusable-release-gate.yml and reusable-latest-tag.yml so repositories call the gate with a few lines instead of copying it. See README, 'Reusable workflows'. CI of this repository runs both against the checkout under test.

v0.2.0

Adds expect.latest_tag (newest tag of a repository, numeric order) and expect.env (environment variable, optionally cut with a regex) for a pre-release gate and a weekly check of README pins. See CHANGELOG.md.

v0.1.1

Fixed: a claim regex with alternation crashed (group 1 was None); the first group that matched is used. Found by the first roll-out.

v0.1.0

First release. .claims.json checker: exact/at-least/at-most matches; expectations from a command, a file regex, a value or a git tag; text/markdown/json/github output; composite action; exit 0/1/2. See README and CHANGELOG.