docs: close the reference conformance gaps

Andrew Dunnrequested to merge
chore/conformity-close-out into main

Adds the CLAUDE.md shim (AGENTS.md already existed), CONTRIBUTING.md, CHANGELOG.md (honest stubs from the real v0.1.0/v0.2.0 tags), CODE_OF_CONDUCT.md, and a Severity section in SECURITY.md.

Content stamped from gitlab-com/public-sector/reference templates, adapted to what is true of this repo (advisory CODEOWNERS, no merge-request template). Remaining audit failures after this: encryption (FIPS in build config) and osv-verdict adoption, both tracked separately.

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