fix(skill-evals): correct blocked drop reason

[apraba05]

Summary

• Correct mergeable-but-blocked, non-review drops from gate:G5-conflict to gate:G5, matching the authoritative candidate rules.
• Add a regression fixture that keeps gate:G5-conflict reserved for genuine merge conflicts.
• Update the quick-merge suite documentation and case count.

Type of change

• Other: skill-eval fixture and behavioral prompt correction

Test plan

• uv run --project tools/skill-evals pytest tools/skill-evals/tests -q passes
• The affected quick-merge stage-3 fixture suite was exercised successfully
• A new regression fixture is included for the corrected behavior
• Targeted prek and commit-time hooks pass
• prek run --all-files passes (the local one-vCPU runner exceeded its bounded execution window; CI will run the full repository gates)

Linked issues

Closes #949

Notes for reviewers

The new case isolates mergeable=true, mergeable_state=blocked, and reviewDecision=APPROVED. The prior fixture text mislabeled this non-conflict state as gate:G5-conflict; gate:G5 is the existing non-approval blocker code.