fix(ci): remove duplicate concurrency block in proofs.yml (#207)

The proofs workflow had two top-level concurrency: keys. YAML's last-wins
semantics meant the second block (cancel-in-progress: true unconditional)
overrode the brief-conformant block PR #200 added at the top of the file,
canceling main and scheduled runs along with PR runs. Removing the
preexisting duplicate restores the correct behaviour: PR runs cancel on
supersede, main / scheduled / tag runs are preserved.

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: avrabe

.github/workflows/proofs.yml

@@ -28,11 +28,6 @@ on:
   pull_request:
     branches: [main]
 
-# Avoid duplicate runs on PR + push for the same SHA.
-concurrency:
-  group: proofs-${{ github.workflow }}-${{ github.ref }}
-  cancel-in-progress: true
-
 jobs:
   lean:
     name: Lean proof typecheck (lake build)