Fix the drop_reason mismatch in the quick-merge stage-3 fixture

[justinmclean]

Summary

A pr-management-quick-merge fixture assigns drop_reason: gate:G5-conflict to
a case that is not a conflict, so the step-4 summary would misattribute why the
PR was dropped.

Background

candidate-rules.md defines gate:G5-conflict as a genuine merge conflict,
meaning mergeable == false. The fixture applies it to a case with
mergeable == true and mergeable_state == blocked: the PR merges cleanly but
a branch protection or check is blocking it. Because the fixture is what the
eval grades against, the wrong reason is locked in as correct.

Where to look

• evals/pr-management-quick-merge/stage-3-merge-readiness/fixtures/system-prompt.md -
  the drop_reason assignment.
• skills/pr-management-quick-merge/candidate-rules.md - the authoritative gate
  codes. Find the one describing a blocked-but-mergeable PR.
• grep -rl 'G5-conflict' tools/skill-evals/evals/pr-management-quick-merge/
  finds the three files that mention it.

Acceptance criteria

• The fixture's drop_reason matches candidate-rules.md for a
  mergeable-but-blocked PR.
• Any expected.json asserting the old reason is updated.
• The pr-management-quick-merge eval suite passes.

Estimated effort

~2 hours for someone new to the codebase.

[rekha0suthar]

I'd like to work on this — starting now.

[justinmclean]

Thanks @rekha0suthar all yours

[rekha0suthar]

Releasing this — redirecting my effort elsewhere. Feel free for someone else to pick it up.

[apraba05]

I’d like to work on this. I verified the issue is currently unassigned and found no matching open PR. I’ll align the mergeable-but-blocked fixture with the authoritative gate in candidate-rules.md, update any affected expected output, and run the focused quick-merge eval suite. Could you assign it to me?

[justinmclean]

Thanks @apraba05, you're welcome to work on it. If you have any questions, please ask.