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/runner/action_cache_offline_mode.go

@@ -1,45 +0,0 @@
-// Copyright 2024 The Gitea Authors. All rights reserved.
-// Copyright 2024 The nektos/act Authors. All rights reserved.
-// SPDX-License-Identifier: MIT
-
-package runner
-
-import (
-	"context"
-	"io"
-	"path"
-
-	git "github.com/go-git/go-git/v5"
-	"github.com/go-git/go-git/v5/plumbing"
-)
-
-type GoGitActionCacheOfflineMode struct {
-	Parent GoGitActionCache
-}
-
-func (c GoGitActionCacheOfflineMode) Fetch(ctx context.Context, cacheDir, url, ref, token string) (string, error) {
-	sha, fetchErr := c.Parent.Fetch(ctx, cacheDir, url, ref, token)
-	gitPath := path.Join(c.Parent.Path, safeFilename(cacheDir)+".git")
-	gogitrepo, err := git.PlainOpen(gitPath)
-	if err != nil {
-		return "", fetchErr
-	}
-	refName := plumbing.ReferenceName("refs/action-cache-offline/" + ref)
-	r, err := gogitrepo.Reference(refName, true)
-	if fetchErr == nil {
-		if err != nil || sha != r.Hash().String() {
-			if err == nil {
-				refName = r.Name()
-			}
-			ref := plumbing.NewHashReference(refName, plumbing.NewHash(sha))
-			_ = gogitrepo.Storer.SetReference(ref)
-		}
-	} else if err == nil {
-		return r.Hash().String(), nil
-	}
-	return sha, fetchErr
-}
-
-func (c GoGitActionCacheOfflineMode) GetTarArchive(ctx context.Context, cacheDir, sha, includePrefix string) (io.ReadCloser, error) {
-	return c.Parent.GetTarArchive(ctx, cacheDir, sha, includePrefix)
-}