ci: add release-gate pipeline, fix CI duplication, add OSS playbook docs
- Add release-gate.yml: 11-job pipeline (lint, tests, Docker build/smoke,
Trivy scan, dep audit, audit checks, e2e API+UI, PR summary comment)
- Fix CI double-trigger on release branches (push + PR)
- Add CODE_OF_CONDUCT.md, SECURITY.md, PR template
- Add docs: git-workflow, architecture (ADR), release, operations, community
- Add profiles/fastapi-vue for automated audit checks
- Add docs/PROCESSES.md: index of 19 available processes
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