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/testdata/local-remote-action-overrides/config.yml

@@ -1,3 +0,0 @@
-local-repositories:
-  https://github.com/nektos/test-override@a: testdata/actions/node24
-  nektos/test-override@b: testdata/actions/node24

act/runner/testdata/local-remote-action-overrides/push.yml

@@ -1,9 +0,0 @@
-name: basic
-on: push
-
-jobs:
-  build:
-    runs-on: ubuntu-latest
-    steps:
-      - uses: https://github.com/nektos/test-override@a
-      - uses: nektos/test-override@b