fix: report one absent file as one failure, not three to five

Adds require_file, an eleventh primitive: a presence gate for single-subject-file standards. It asserts existence exactly like check_file_exists; when the file is absent it stops the standard after that one failure. contributing, changelog, code-of-conduct, conformance-config, codeowners gate on it; ai-assisted scopes with applies_when_file 'CONTRIBUTING.md' since the existence claim belongs to contributing. applies_when_file hardened to exit with report's verdict so a gate after an assertion cannot swallow a recorded failure. Verify blocks regenerated.

Measured on kaniko/manifold/postern/assay/egress: 87 failure lines to 52, zero defects hidden; self-check stays 20/20.