PR hygiene: issue forms, labels and a CI check, as in ValueRanges #68

The repository tracks no work as issues: 66 pull requests, no issue, no milestone, and no labels of its own beyond GitHub's defaults. Adopt CodoMetis.ValueRanges' scheme:

• issue forms (Bug, Enhancement, Epic), blank issues off, security reports sent to the advisory form;
• a pull request template (Closes #n, Problem, Change, Tests);
• ValueRanges' labels: one type, severity/* on a bug, package/*, area/*, silent, breaking, found-by/prober, stacked, epic;
• a CI check on every pull request into main or a release branch that fails a missing type, severity or closing reference, and here also a branch whose <type>/ disagrees with its type label, a pull request into release/<version> without that milestone, and breaking into anything but a major. Under decision 92 it is a C# tool project, not ValueRanges' bash script.

Done when the forms, template, labels and check are in, decision 109 records them, and CONTRIBUTING and AGENTS.md describe them.