Remove dead code from act/ (#971)

Removes code that whole-program reachability analysis (`deadcode` from `golang.org/x/tools`) confirmed unreachable, plus the `act/workflowpattern` package which no file outside its own directory imports.

- `act/common/draw.go` — CLI box-drawing helpers left over from nektos/act's dropped CLI
- `act/common/file.go` — `CopyFile`/`CopyDir` package-level helpers (container types have their own `CopyDir` methods, kept)
- `act/common/executor.go` — `Warning` type and `Warningf`. The `case Warning:` arm in `(Executor).Then`'s type switch was dead too (no code ever constructed a `Warning`); the switch is replaced with `if err != nil { return err }`
- `act/lookpath/env.go` — `LookPath` no-arg wrapper and `defaultEnv` struct. Only `LookPath2(file, env)` was used externally; the `Env` interface is kept
- `act/runner/action_cache_offline_mode.go` — `GoGitActionCacheOfflineMode` wrapper, never instantiated
- `act/workflowpattern/` — entire package, never imported

Net `-943` lines.

---
This PR was written with the help of Claude Opus 4.7

---------

Co-authored-by: Nicolas <bircni@icloud.com>
Reviewed-on: https://gitea.com/gitea/runner/pulls/971
Reviewed-by: Nicolas <bircni@icloud.com>
Co-authored-by: silverwind <me@silverwind.io>
Co-committed-by: silverwind <me@silverwind.io>

act/lookpath/env.go

@@ -4,18 +4,6 @@
 
 package lookpath
 
-import "os"
-
 type Env interface {
 	Getenv(name string) string
 }
-
-type defaultEnv struct{}
-
-func (*defaultEnv) Getenv(name string) string {
-	return os.Getenv(name)
-}
-
-func LookPath(file string) (string, error) {
-	return LookPath2(file, &defaultEnv{})
-}