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/runner/runner.go

@@ -73,6 +73,7 @@ type Config struct {
 	EventJSON             string                       // the content of JSON file to use for event.json in containers, overrides EventPath
 	ContainerNamePrefix   string                       // the prefix of container name
 	ContainerMaxLifetime  time.Duration                // the max lifetime of job containers
+	CleanWorkdir          bool                         // remove host executor workdir on teardown
 	DefaultActionInstance string                       // the default actions web site
 	PlatformPicker        func(labels []string) string // platform picker, it will take precedence over Platforms if isn't nil
 	JobLoggerLevel        *log.Level                   // the level of job logger
@@ -91,6 +92,17 @@ func (c Config) GetToken() string {
 	return token
 }
 
+// DefaultActionURL returns the host used for implicit remote actions.
+func (c Config) DefaultActionURL() string {
+	if c.DefaultActionInstance != "" {
+		return c.DefaultActionInstance
+	}
+	if c.GitHubInstance != "" {
+		return c.GitHubInstance
+	}
+	return "github.com"
+}
+
 type caller struct {
 	runContext *RunContext