refactor: remove unreachable runner code (#1179)

Remove inherited act APIs, configuration branches, and test seams that neither the daemon nor exec can reach. Constant-fold settings both entry points already enforce and consolidate duplicate runner paths.

Major removals:

- Unwired custom action-cache and local-repository-cache implementations.
- Legacy matrix, platform, input, container-reuse, logging, Git remote, and action-replacement configuration paths.
- Unused Docker socket, container network, tar-copy, and platform PTY wrappers.
- Single-implementation filesystem, environment, runner, and expression abstractions.
- Duplicated step-container, command-logging, credential, reusable-workflow, and execution paths.
- Generated client mock boilerplate, obsolete fixtures, test-only seams, stale wrappers, and commented-out code.

This removes 2844 net Go lines while retaining Gitea RPC, event, matrix, input, cache, artifact, action, reusable workflow, Docker, host, exec, and release behavior.

Assisted by Codex (GPT-5).

Reviewed-on: https://gitea.com/gitea/runner/pulls/1179
Reviewed-by: bircni <bircni@icloud.com>
Co-authored-by: silverwind <me@silverwind.io>

act/lookpath/lp_unix.go

@@ -33,7 +33,7 @@ func findExecutable(file string) error {
 // directories named by the PATH environment variable.
 // If file contains a slash, it is tried directly and the PATH is not consulted.
 // The result may be an absolute path or a path relative to the current directory.
-func LookPath2(file string, lenv Env) (string, error) {
+func LookPath2(file string, env map[string]string) (string, error) {
 	// NOTE(rsc): I wish we could use the Plan 9 behavior here
 	// (only bypass the path if file begins with / or ./ or ../)
 	// but that would not match all the Unix shells.
@@ -45,7 +45,7 @@ func LookPath2(file string, lenv Env) (string, error) {
 		}
 		return "", &Error{file, err}
 	}
-	path := lenv.Getenv("PATH")
+	path := getenv(env, "PATH")
 	for _, dir := range filepath.SplitList(path) {
 		if dir == "" {
 			// Unix shell semantics: path element "" means "."