docs: add OSS-hygiene files (SECURITY, CHANGELOG, SUPPORT, CONTRIBUTING, CODEOWNERS, README, templates)

Public-facing docs + PR and issue templates. README rewritten for
external readers; CONTRIBUTING documents the scope-tight posture
and commit style; SECURITY.md establishes the disclosure process
with 72h triage / 7d-P0 / 30d-P1 SLA; CODEOWNERS gives the solo
maintainer review blocking on every module.