daily-fix: Step 10d tasks/* merge-conflict default + far-behind branch

Overview / Motivation

Auto-filed by the /daily three-route problem sweep (2026-06-30), route 2 (behavior/logic change → independent review pipeline).

Goal

Make the scratch-worktree-detached-at-origin/main surgical merge the DEFAULT Step-10d path for workflow-fix/small-diff tasks when BEHIND exceeds a threshold (skip the doomed --rebase); add .gitattributes merge=union for tasks/**/events.jsonl (append-only) and -X no-renames/exclude tasks/* from the merge so concurrent task folders never conflict; exclude Step-5a-synced workflow-surface files from the Guard-3 out-of-scope check.

Workflow gap

• Bug observed: Step 10d worktree->main merge repeatedly conflicts on foreign concurrent-session tasks/* files (events.jsonl content + rename/delete conflicts from concurrent status moves) and re-imports stale task-folder snapshots, forcing ad-hoc scratch-worktree-detached-at-origin/main + cherry-pick recovery reconstructed by hand every time (issues 739/748/749/752/753/755/762/764). Far-behind experiment branches (#697 3771 behind) additionally fail Guard-3 because Step-5a spec-freshness sync copied now-stale workflow-surface files, parking for a manual rebase.
• Evidence: Dominant friction — confirmed by /daily miners across batches 01/04/05/06/08/10.
• Confidence (emitter): medium

Scope / surfaces

• Primary target: .claude/skills/issue/SKILL.md
• Grep the workflow surface for the pattern before editing and update every hit; list them in the plan.

Constraints / invariants

• Workflow-surface / code fix per the target; keep ruff + workflow_lint + the relevant tests green.
• The planner may deflect with a reasoned no-change report if the gap is already closed on main.

Provenance

• workflow_fix_target: .claude/skills/issue/SKILL.md
• fingerprint: 9e057d95ad05
• source: /daily route-2 (2026-06-30)