[JUM-910][Chore] Stop test-map.json from conflicting on every PR

[web2solutions]

Summary

• test-map.json is fully generated (deterministic filesystem walk + sort), so any two branches touching test files diverge and textually conflict on merge even when neither is wrong — this hit every PR in the docs epic (JUM-895/896/907/908 each needed a manual git merge origin/dev + conflict resolution here).
• ci-cd/setup-git-merge-drivers.js (new, run from prepare — self-registers on bun install in every clone/worktree, no extra onboarding step) registers a git merge driver for test-map.json that keeps "ours" instead of a textual 3-way merge, so a conflict here can never block a merge.
• test-map:check moves into ci:gate:static (previously only in the full/strict dev/main matrix), so every branch-gate preflight — including plain task branches — fails closed immediately on the resulting staleness, forcing bun run test-map:generate before it can be pushed.

Why "keep ours" instead of regenerating inside the driver

Verified this empirically, not by assumption: a first design tried to regenerate the file inside the merge driver. Reproducing a real two-branch conflict showed it silently produced an incomplete manifest — missing the other branch's new test file — because a merge driver is not guaranteed the full merged tree is checked out to disk yet. The shipped design avoids that hazard entirely by never trusting driver-time disk state; it defers to test-map:check running after the merge fully completes.

Test plan

• Reproduced a real test-map.json conflict between two scratch branches twice (once per driver design) to prove the fix, not just plausibility.
• bun run test-map:check — passes clean.
• bun run lint (root) — clean.
• bun run ci:gate:static — all green, including the new test-map:check step.
• Confirmed the merge driver is visible from a sibling worktree (shared git config), so no extra setup is needed once this merges and clones run bun install.

🤖 Generated with Claude Code

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Note

Low Risk
Developer workflow and CI gate wiring only; correctness still enforced by test-map:check, with no runtime or auth changes.

Overview
Stops merge-blocking conflicts on generated test-map.json when parallel branches both touch tests, without silently shipping a wrong manifest.

.gitattributes assigns test-map.json a custom merge driver (keep-ours-test-map) so Git keeps the current branch’s copy instead of doing a textual 3-way merge.

ci-cd/setup-git-merge-drivers.js registers that driver in local git config (true — no in-driver regeneration). It runs from prepare after husky, so bun install sets it up on every clone/worktree.

ci:gate:static now includes test-map:check, so branch preflights fail until someone runs test-map:generate on a fully checked-out tree after a merge that left a stale map.

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