feat: add backport automation skills

[lokanandaprabhu]

Summary

• Adds three backport skills to automate the RHDH plugin backport process (currently fully manual, inherited from backstage/community-plugins)
• backport-auto — Fully automated end-to-end: detect plugin, reset workspace, cherry-pick (with AI conflict resolution), create sequential PRs, handle Version Packages, /publish + merge overlays, changelog PR
• backport-create — Semi-manual: creates backport PRs and stops for user review
• backport-finish — Completes the workflow after manual PR merges (Version Packages, overlays, changelog)

Files

• skills/backport-auto/ — SKILL.md + README.md + 8 reference files
• skills/backport-create/ — SKILL.md + README.md (reuses backport-auto references)
• skills/backport-finish/ — SKILL.md + README.md (reuses backport-auto references)
• README.md — Added Backport section

Known Gaps (pending team input)

1. Concurrent backport race condition

The workspace/{plugin} branch is shared across all releases. If two people backport the same plugin to different releases simultaneously (e.g., 1.9 and 1.10), the second workspace reset clobbers the first. Waiting for team guidance on approach (release-scoped branches, locking, etc.).

2. Branch pre-creation for new plugins/releases

workspace/{plugin} and {plugin}/release-x.y branches may not exist for new plugins or new release versions. Need clarification on whether the skill should create them or if they must be pre-created manually.

Test plan

• Dry-run with an already-merged PR to validate detection and prerequisite logic
• End-to-end test with a real backport PR once available
• Validate /publish + metadata version mismatch fix flow on overlays repo

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