refactor: remove molecule_ci_guard — adds complexity, no value

The cross-runner cancellation guard was killing healthy molecule slots
when unhealthy slots failed, defeating fail-fast: false. With fail-fast:
false and no guard, slots on healthy runners complete independently.

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Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

docs/tech/ci-cd-workflow.md

@@ -478,15 +478,6 @@ python -m devx.molecule.distribute_molecule --list
 python -m devx.molecule.distribute_molecule --list-platforms
 ```
 
-### `molecule_ci_guard.py`
-
-Runs molecule tests sequentially while polling the Gitea API for other runner
-failures. Aborts early if another runner fails the same job.
-
-```bash
-python -m devx.molecule.molecule_ci_guard [--roles-root <dir>] pair1 pair2 ...
-```
-
 ### `validate_commit_msg.py`
 
 Validates commit messages. On feature branches: conventional commits only