fix: stop a stalled docker daemon from hanging jobs and the e2e suite (#1404)

When the docker daemon stopped answering on https://gitea.com/gitea/runner/actions/runs/1204971, unbounded exec runs hung the e2e suite until the 20m `go test` panic, which skipped teardown. Exec runs and teardown are now bounded, and attached exec streams, including the runner's user ID probe, close with their context.

CLI daemons now stop with SIGTERM, so job volumes no longer leak.

_Written by Claude (Opus 5.5)._

---------

Co-authored-by: bircni <[email protected]>
Reviewed-on: https://gitea.com/gitea/runner/pulls/1404
Reviewed-by: bircni <[email protected]>
Co-authored-by: silverwind <[email protected]>

e2e/testdata/workflows/cli_labels.yml

@@ -1,7 +0,0 @@
-name: cli-labels
-on: push
-jobs:
-  hello:
-    runs-on: e2e-override
-    steps:
-      - run: echo e2e-override-ran