A .gitattributes merge driver is retroactively inert: every PR older than the declaration still conflicts #3308

Measurement

Seven open PRs could not even be updated onto main today — gh pr update-branch refused each with "Cannot update PR branch due to conflicts". The conflict surface, measured across all seven:

docs/audits/impl-estimates.jsonl should never conflict. It is strictly append-only — every commit touching it is N insertions, 0 deletions — and #3256 (768e740b1) declared exactly the right thing:

docs/audits/*.jsonl merge=union

It still conflicted.

Mechanism

A .gitattributes merge-driver declaration is retroactively inert. Git reads merge attributes from the side being merged into, not from the side being merged in. A branch cut before the rule landed carries a .gitattributes without it, so the driver is never consulted.

Measured on agent/T-2 (#3005):

$ git show origin/agent/T-2:.gitattributes
docs/roadmaps/roadmap.yaml merge=roadmap      <- no jsonl rule

$ git merge-base --is-ancestor 768e740b1 origin/agent/T-2 ; echo $?
1                                              <- branch predates the declaration

$ git merge origin/main
CONFLICT (content): Merge conflict in docs/audits/impl-estimates.jsonl

The blobs themselves merge perfectly — git merge-file --union on the three stages returns rc=0, 50 rows, 0 markers, every line valid JSON. Nothing was wrong with the data or the driver; the attribute simply never bound.

This is the "declared fix that cannot fire" class. The repo has had the correct declaration since #3256 and every PR older than it kept conflicting anyway, silently, with no signal that the declaration was not in play.

Fix

scripts/ci_resolve_dirty.sh already owns this class for roadmap.yaml: it registers merge.roadmap.driver per invocation so nothing leaks into the shared .git. Bind union the same way:

git -c core.attributesFile=<tmp with `docs/audits/*.jsonl merge=union`> merge ...

core.attributesFile is the lowest-priority attribute source, so it applies only where the branch has no rule of its own — a branch that already carries the declaration is unaffected. Nothing shared is written.

Verified on #3271, whose merge had failed minutes earlier:

merge rc=0
Auto-merging Makefile
Auto-merging docs/audits/impl-estimates.jsonl
Auto-merging docs/roadmaps/roadmap.yaml

49 rows, 0 invalid, 0 duplicate; check_roadmap_ids_unique.sh and check_roadmap_sorted.sh both PASS.

Drained under this diagnosis

#3005, #3271, #3278, #3281 pushed out of CONFLICTING; seven more (#3205, #3238, #3246, #3249, #3259, #3265, #3270) updated onto main. Remaining conflicts (#3134, #3245, #3248) are genuine ci.yml content edits and are not this class.