ci: report PR compatibility breaks and qualification acknowledgements #4487

User Story

I review OpenShell pull requests and release qualification reports. I need breaking changes reported before merge and qualification findings to show whether the introducing PR acknowledged the risk, with concise descriptions I can act on.

Description

Run the advisory compatibility reviewer in Branch Checks. When it finds incompatibilities, publish a fresh PR comment on each run; do not update an earlier comment, and do not comment when there are no findings.

For both branch checks and release qualification, have the reviewer inspect code first, then use read-only gh commands to find the relevant PR and inspect its discussion. Mark each finding ACKED, NOT ACKED, or UNKNOWN. Keep the readable report to concrete changes, affected consumers, required actions, and PR links.

Context

The existing reviewer runs only for tagged releases and produces reports that are too abstract to scan easily. Acknowledgement records awareness of a specific risk and does not override release policy. The reviewer remains advisory.

Definition of Done

• Branch Checks runs the compatibility reviewer against the prospective merge tree.
• Each mirrored PR run with findings creates a fresh comment; clean runs create none.
• Comments are skipped when the reviewed PR head is outdated or the PR has closed.
• Qualification findings identify the introducing PR and its acknowledgement status.
• Both paths use code-first review followed by read-only GitHub CLI lookups.
• Readable reports are concise and omit evidence sections.