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 <me@silverwind.io>
Reviewed-on: https://gitea.com/gitea/runner/pulls/970
Reviewed-by: silverwind <2021+silverwind@noreply.gitea.com>

act/artifactcache/handler.go

@@ -431,6 +431,7 @@ func (h *Handler) upload(w http.ResponseWriter, r *http.Request, params httprout
 	}
 	if err := h.storage.Write(cache.ID, start, r.Body); err != nil {
 		h.responseJSON(w, r, 500, err)
+		return
 	}
 	h.useCache(id)
 	h.responseJSON(w, r, 200)