docs: add operational notes to AGENTS.md

Author: xnoto

AGENTS.md

@@ -21,6 +21,16 @@ make test
 
 This automatically fetches the canonical config if not present.
 
+## CI/CD
+
+This repo uses the shared `opentofu.yml` workflow from `shared-workflows`. It runs on `ubuntu-latest` with the `tfroot-runner` container from GHCR.
+
+### Failure Modes
+
+**"manifest unknown" error:** The `tfroot-runner:latest` image doesn't exist in GHCR. Check if the `images` repo Build workflow succeeded.
+
+**Pre-commit failures:** If hooks fail unexpectedly, the canonical config may have changed. Delete `.pre-commit-config.yaml` locally and re-run `make test` to fetch the latest.
+
 ## Related Repositories
 
 - `images` - Contains tfroot-runner image and canonical pre-commit config