ci(coq-build): run coqorg/coq container as root to fix actions/checkout EACCES (#236)

## Summary

The Coq merge-oracle workflow (`coq-build.yml`) was failing across every
`formal/` PR (including #234 which had to bypass) with:

```
Error: EACCES: permission denied, open '/__w/_temp/_runner_file_commands/save_state_…'
```

## Root cause

`coqorg/coq:8.18` defaults to the non-root `coq` user (UID 1000).
`actions/checkout` writes save_state files to
`/__w/_temp/_runner_file_commands` which is mounted from the runner host
— the `coq` user lacks write permission there. The job dies in
checkout's post-step before `coq_makefile` runs.

## Fix

`container.options: --user root` runs the container as root. The shared
workspace paths become writable; the in-container build environment is
otherwise unchanged (root inside the container maps to the runner host's
runner user via the Actions sandbox, not host-root).

## Why this matters

Per `[[feedback_proof_pr_build_oracle_is_only_truth]]` the Coq workflow
is the *only* merge oracle for `formal/` PRs. If the workflow itself
can't run, the hard gate is effectively bypassed.

## Test plan

- [ ] `Coq build — formal/_CoqProject` goes green on this PR
- [ ] `Print Assumptions of load-bearing theorem` step actually executes
(visible in run log)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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