[Framework] Improve evaluate.sh branch target detection — query actual PR merge targets

[Kanevry]

Problem

evaluate.sh infers PR target branch by checking if dev or develop branches exist (lines 65-78). This heuristic can be wrong — a branch may exist but not be the PR target (archived/legacy).

Proposed Fix

Query recent merged PRs to determine actual target branch:
```bash
gh pr list -R $REPO --state merged --limit 5 --json baseRefName --jq '.[].baseRefName' | sort | uniq -c | sort -rn | head -1
```

References

• Audit finding from skills session 2026-03-27
• react-router contribution confirmed this is a real issue (PRs target dev, not main)