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/user/cli-commands.md

@@ -531,29 +531,3 @@ Options:
 - `--list-platforms` — list all platforms, one per line
 - `--roles-root <dir>` — roles root directory for multi-role repos (default:
   `ansible/roles`)
-
-### `devx molecule guard`
-
-Run molecule tests sequentially with CI failure polling. A background thread
-polls the Gitea API. If any other molecule matrix runner reports failure, the
-current molecule subprocess is killed and this runner exits early with code 1.
-
-```bash
-devx molecule guard pair1 pair2 pair3
-devx molecule guard --roles-root ansible/roles pair1 pair2
-```
-
-Each pair is encoded as:
-- **Single-role (4-part):** `scenario|platform_name|platform_image|platform_command`
-- **Multi-role (5-part):** `role|scenario|platform_name|platform_image|platform_command`
-
-Options:
-- `--roles-root <dir>` — roles root directory for multi-role repos
-
-Environment variables:
-- `GITEA_URL` — base URL of the Gitea instance
-- `CI_GITEA_TOKEN` — API token with repo access
-- `RUN_ID` — workflow run ID (`GITHUB_RUN_ID`)
-- `JOB_NAME` — base job name (`GITHUB_JOB`)
-- `MATRIX_INDEX` — current matrix index (runner-index)
-- `GITEA_REPOSITORY` — repository in `owner/repo` format