Fix host cleanup, volume allowlist, cache upload, and action host edge cases (#970)

## Summary
- prevent host-mode execution from deleting caller-owned workdirs
- harden `valid_volumes` checks against `..` and symlink escapes
- return immediately after artifact cache upload write failures
- default implicit remote action clone hosts to `GitHubInstance`/`github.com`

Authored with assistance from OpenAI Codex GPT-5.

---------

Co-authored-by: silverwind <[email protected]>
Reviewed-on: https://gitea.com/gitea/runner/pulls/970
Reviewed-by: silverwind <[email protected]>

internal/app/run/runner.go

@@ -363,6 +363,7 @@ func (r *Runner) run(ctx context.Context, task *runnerv1.Task, reporter *report.
 		EventJSON:             string(eventJSON),
 		ContainerNamePrefix:   fmt.Sprintf("GITEA-ACTIONS-TASK-%d", task.Id),
 		ContainerMaxLifetime:  maxLifetime,
+		CleanWorkdir:          true,
 		ContainerNetworkMode:  container.NetworkMode(r.cfg.Container.Network),
 		ContainerOptions:      r.cfg.Container.Options,
 		ContainerDaemonSocket: r.cfg.Container.DockerHost,