Adopt the ValueRanges tracking culture: issue forms, PR template, hygiene workflow, CONTRIBUTING labels and branches, SECURITY rows #40

Labels, milestones and the pinned roadmap were created on 2026-10-18 (this issue's parent). The repository files that make the culture enforceable are still ValueRanges-only; port them:

• .github/ISSUE_TEMPLATE/{bug,enhancement,epic}.yml and config.yml, with this repository's package and area labels.
• .github/PULL_REQUEST_TEMPLATE.md (Problem / Change / Tests, Closes #n, labels and milestone).
• .github/workflows/pr-hygiene.yml and .github/scripts/pr-hygiene.sh: exactly one type label, severity on bugs, a closing reference.
• CONTRIBUTING.md: the labels section (the roadmap issue's legend, verbatim) and a "Branches and releases" section stating the two lines, release/10.x cut at the v10.0.0 tag, net11 merge-forward until then, main-first then git cherry-pick -x after, and the silent-fix rule: on both lines a silent fix ships as a minor with a Changed answers section at the top of the changelog entry.
• SECURITY.md: two supported rows, one per line.
• AGENTS.md: the same rules in the agent guide.

Done when a PR without a type label fails the hygiene check here as it does in ValueRanges.