Add ACP cluster comparison report

[dutifulbob]

Summary

The earlier manual ACP duplicate groups and the current pr-search-cli issue clusters do not line up cleanly.
This change adds a short report that compares them directly and states which grouping looks more faithful to the actual PRs.
It matters because we are likely going to keep updating this comparison as we refine the curated groups.

What Changed

The report compares the two earlier manual prtags groups against the current pr-search-cli cluster output.
It records the exact mismatches, then gives an objective read on which grouping looks more correct.
The main conclusion is that the manual sessions_spawn cluster looks better, while the /acp duplicate set is probably broader than either current narrow cluster.

• Added docs/2026-04-20-acp-cluster-comparison-report.md
• Recorded the two manual prtags groups
• Recorded the observed pr-search-cli cluster results
• Documented the recommended update: keep the sessions_spawn cluster and broaden the /acp cluster to include #65248

Testing

I only changed documentation here.
I ran the repository doc checks and whitespace check.

• npx --yes @simpledoc/simpledoc@0.1.6 check
• git diff --check

Risks

The risk is low because this is only a report.
The only real risk is that the judgment in the report may need to change as we gather more evidence or update the clusters.

Follow-ups

We will likely keep updating this PR as the cluster comparison evolves.