daily-fix: merge=union for tests/sparse_cones.txt

Overview / Motivation

Auto-filed by /daily 2026-07-19 (route 2) from transcript-mined problems (see evidence in ## Provenance).

Goal

Add a merge=union gitattribute for tests/sparse_cones.txt so append-only cone registrations stop producing hard merge conflicts.

Workflow gap

• Bug observed: tests/sparse_cones.txt conflicted in TWO separate merges the same day (sessions 98ff0f37 #1481 @ 10:30 UTC and 44e00194 #1417 @ 18:42 UTC — both sides appended cones), each needing a manual union resolution inside a merge/conflict round.
• Why it is a workflow gap: the file is an append-only registry that conflicts by construction under concurrent appends; the repo already uses union merge for exactly this class (events.jsonl/comments.jsonl per .gitattributes, #787; eval_results/INDEX.md added by #1534 on 2026-07-18).
• Confidence (emitter): medium-high
• verified-at-filing: cat .gitattributes → union-merge mechanism present (#787 header; JSONL logs covered); grep -c sparse_cones .gitattributes → 0 (absence claim, in-target 0-hit is the evidence). Two same-day conflict incidents anchored in transcripts (2026-07-19). Ordering caveat for the plan: union merge concatenates BOTH sides' appended lines — verify sparse-checkout cone semantics tolerate duplicate/reordered lines (they should; the registry is read as a set).

Proposed change (candidate diff sketch — refine in planning)

+ tests/sparse_cones.txt merge=union

Scope / surfaces

• Primary target: .gitattributes
• Verify duplicate-line tolerance in scripts/new_worktree.sh's consumer of tests/sparse_cones.txt.

Constraints / invariants

• Workflow-surface rules apply where the target is workflow surface; scripts/workflow_lint.py --check-asks passes; ruff on touched files passes.
• Recursion guard applies where tagged wf-fix (workflow_fix_target Provenance line below).

Provenance

• sha-verify (filing-time, #1467): 98ff0f37 cited in commit context does NOT resolve as a commit in this repo at filing time — treat as a transcript/session reference, not a commit.

• sha-verify (filing-time, #1467): 44e00194 cited in commit context does NOT resolve as a commit in this repo at filing time — treat as a transcript/session reference, not a commit.

• workflow_fix_target: .gitattributes

• fingerprint: 44d3a4598f5c

Mined evidence: two same-day union-resolved conflicts on tests/sparse_cones.txt (#1481 and #1417 merge rounds, 2026-07-19).