ci(proofs): bisect — isolate taiki install-action step

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

Author: claude

.github/workflows/dogfood-proofs-ci.yml

@@ -1,6 +1,6 @@
 # SPDX-License-Identifier: MPL-2.0
 # Gates the ECHIDNA dogfood proof corpus (proofs/{coq,lean,agda}) in CI.
-# (Bisecting a startup_failure: restoring jobs one at a time.)
+# (Bisecting a startup_failure: isolating the taiki-e/install-action step.)
 name: Dogfood Proof Corpus
 
 on:
@@ -32,17 +32,10 @@ jobs:
     steps:
       - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
 
-      - name: Install Coq
-        run: |
-          set -euo pipefail
-          sudo apt-get update -qq || sudo apt-get update -qq --fix-missing
-          sudo apt-get install -y --fix-missing coq
-          coqc --version
-
       - name: Install just
         uses: taiki-e/install-action@e49978b799e49ff429d162b7a30601a569ab6538 # v2.81.1
         with:
           tool: just@1.51.0
 
-      - name: Type-check Coq corpus
-        run: just proofs-coq
+      - name: Smoke
+        run: just --version