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/runner/step_test.go

@@ -157,8 +157,7 @@ func TestSetupEnv(t *testing.T) {
 	sm.On("getStepModel").Return(step)
 	sm.On("getEnv").Return(&env)
 
-	err := setupEnv(context.Background(), sm)
-	assert.NoError(t, err) //nolint:testifylint // pre-existing issue from nektos/act
+	setupEnv(context.Background(), sm)
 
 	// These are commit or system specific
 	delete((env), "GITHUB_REF")
@@ -213,12 +212,7 @@ func TestIsStepEnabled(t *testing.T) {
 
 		return &stepRun{
 			RunContext: &RunContext{
-				Config: &Config{
-					Workdir: ".",
-					Platforms: map[string]string{
-						"ubuntu-latest": "ubuntu-latest",
-					},
-				},
+				Config:      &Config{Workdir: ".", PlatformPicker: func([]string) string { return "ubuntu-latest" }},
 				StepResults: map[string]*model.StepResult{},
 				Env:         map[string]string{},
 				Run: &model.Run{
@@ -295,12 +289,7 @@ func TestIsContinueOnError(t *testing.T) {
 
 		return &stepRun{
 			RunContext: &RunContext{
-				Config: &Config{
-					Workdir: ".",
-					Platforms: map[string]string{
-						"ubuntu-latest": "ubuntu-latest",
-					},
-				},
+				Config:      &Config{Workdir: ".", PlatformPicker: func([]string) string { return "ubuntu-latest" }},
 				StepResults: map[string]*model.StepResult{},
 				Env:         map[string]string{},
 				Run: &model.Run{