Add CLI reference README for microshift-release scripts

[redhat-chai-bot]

Adds plugins/microshift-release/CLI_REFERENCE.md — a standalone CLI reference for running the MicroShift release validation scripts directly, without the AI skill layer.

Covers all phases (0–4) with prerequisites, environment variables, CLI arguments, examples, and a quick-reference cheat sheet.

What's included

• Phase 0 — Pre-Check: precheck.sh subcommands (xyz, nightly, ecrc, enrich)
• Phase 1 — Build Validation: validate.sh for RPM and bootc artifact checks
• Phase 2 — Automated Testing: prow_testing.sh with all 8 actions (preflight through complete)
• Phase 3a — Advisory Promotion: advisory_promotion.sh for bootc stage/prod checks
• Phase 3b — Errata Promotion: errata_promotion.sh for RPM advisory QE sign-off
• Phase 4 — Post-Release: post_release.sh for customer-facing verification

The existing README.md (skills/plugin-focused) is left unchanged.

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AI-generated. Review for accuracy.

@dhensel-rh requested via Chai Bot

[openshift-ci]

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