Add plain-English explanation of PR #280 CI/CD refactor

[Copilot]

PR #280 modernises the CI/CD pipeline but the changes are non-trivial for readers unfamiliar with GitHub Actions. This adds a standalone reference document explaining every change.

What's in pr-280-explanation.md

• Background — brief primer on CI/CD and GitHub Actions for non-practitioners
• Files changed — table covering all 6 files and their purpose
• Pipeline topology — ASCII diagrams comparing old (8-job sequential, ~6 min) vs new (5-job parallel, ~3 min) structure
• Per-change breakdown — covers branch gate consolidation, build caching via lock files, security scan matrix refactor, Docker-local-only build, ZAP action migration, gatekeeper job, Heroku removal, and required branch protection rule follow-up
• Summary table — before/after for all key behaviours

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