Batcher: regenerate derived contract files after merge main (contracts_resolve.sh), stop merges DIRTYing every open PR #5768

Owner: aprender-a3 (batcher). Filed by aprender-flow at aprender-57's request.

Root: contracts/contracts.nt, shapes.ttl and contracts/witness/.json are committed derived files; the contracts gate regenerates and runs git diff --exit-code. Every merge that touches a contracts/*.yaml renames the witness, so every other open PR carrying a contract change goes DIRTY (8 at the #5752 merge). Not a merge-driver case: GitHub's merge ignores .gitattributes drivers (see nt-ticket, #5762 proposal).

Countermeasure (two-way door, minutes to undo): the batcher runs scripts/contracts_resolve.sh (work/5762 @9ff4016841, script-only, tested 8/8 on #5710 and #5672) after each git merge origin/main, instead of hand-resolving. It drops the generated paths, regenerates with the gate's own regen + pv-sat + readme_sync, stages them. Undo: stop calling it.

Touches what the gate accepts: NO. The gate, workflows and the committed files are unchanged; the gate still regenerates and diffs, and stays the judge. (The earlier regen-at-merge proposal that untracked the files DID change gate content and got NO from the 3-lane quorum; this is not that.)

Limit: fixes batched PRs only; a solo PR that goes DIRTY still needs merge main + regen by hand. Time: batcher adopts within 1 h of #5762 landing (it rode batch 1, #5765). Falsifier: a batch of 2 contract-adding PRs folds with 0 manual conflict edits. Related: #5762, #5764.