ci(coq-build): run coqorg/coq container as root to fix actions/checkout EACCES

coqorg/coq:8.18 defaults to the non-root `coq` user (UID 1000). actions/checkout's
post-step writes save_state files to /__w/_temp/_runner_file_commands which is
mounted from the runner host; the coq user lacks write permission there and
the job dies with EACCES before `coq_makefile` runs (see #234 PR run).
`--user root` makes the shared workspace paths writable while leaving the
in-container build environment effectively unchanged (root inside the
container maps to the runner host's runner user via the Actions sandbox).

Without this fix the merge-oracle workflow itself can't run, defeating the
hard-gate guarantee in [[feedback_proof_pr_build_oracle_is_only_truth]].

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

Author: hyperpolymath

.github/workflows/coq-build.yml

@@ -48,6 +48,14 @@ jobs:
     # `make` semantics — "what the maintainer's machine has".
     container:
       image: coqorg/coq:8.18
+      # actions/checkout's post-step writes to /__w/_temp/_runner_file_commands
+      # via Node fs.writeFileSync. The coqorg/coq image defaults to the
+      # non-root `coq` user (UID 1000), but the runner mounts /__w as the
+      # host runner user — EACCES until checkout's saveState. Running the
+      # container as root keeps the workspace-shared paths writable while
+      # leaving the in-container build untouched (root in container is
+      # still mapped to the runner host's runner user, not host-root).
+      options: --user root
 
     steps:
       - name: Checkout repository