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/matrix-with-user-inclusions/push.yml

@@ -1,34 +0,0 @@
-name: matrix-with-user-inclusions
-on: push
-
-jobs:
-  build:
-    name: PHP ${{ matrix.os }} ${{ matrix.node}}
-    runs-on: ubuntu-latest
-    steps:
-      - run: |
-          echo ${NODE_VERSION} | grep 8
-          echo ${OS_VERSION} | grep ubuntu-18.04
-        env:
-          NODE_VERSION: ${{ matrix.node }}
-          OS_VERSION: ${{ matrix.os }}
-    strategy:
-      matrix:
-        os: [ubuntu-18.04, macos-latest]
-        node: [4, 6, 8, 10]
-        exclude:
-          - os: macos-latest
-            node: 4
-        include:
-          - os: ubuntu-16.04
-            node: 10
-
-  test:
-    runs-on: ubuntu-latest
-    strategy:
-      matrix:
-        node: [8.x, 10.x, 12.x, 13.x]
-    steps:
-      - run: echo ${NODE_VERSION} | grep 8.x
-        env:
-          NODE_VERSION: ${{ matrix.node }}