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/ghcontext/github_context_test.go

@@ -104,7 +104,7 @@ func TestSetRef(t *testing.T) {
 				Event:     table.event,
 			}
 
-			SetRef(context.Background(), ghc, "main", "/some/dir")
+			SetRef(context.Background(), ghc, "/some/dir")
 			ghc.SetRefTypeAndName()
 
 			assert.Equal(t, table.ref, ghc.Ref)
@@ -122,7 +122,7 @@ func TestSetRef(t *testing.T) {
 			Event:     map[string]any{},
 		}
 
-		SetRef(context.Background(), ghc, "", "/some/dir")
+		SetRef(context.Background(), ghc, "/some/dir")
 
 		assert.Equal(t, "refs/heads/master", ghc.Ref)
 	})