Rename act_runner to runner (#850)

## Consumer-facing breaking changes

- **Go module path**: `gitea.com/gitea/act_runner` → `gitea.com/gitea/runner`. Anything importing `act/...` or `internal/...` packages (notably Gitea itself) must update imports.
- **Binary name**: `act_runner` → `gitea-runner`. Wrapper scripts, systemd units, init scripts, and documentation referencing the binary by `act_runner` will break.
- **Docker image**: `gitea/act_runner` → `gitea/runner` (incl. `*-dind-rootless` variants). Users pulling `gitea/act_runner:nightly` etc. will get stale images. Note: the image name is `gitea/runner`, not `gitea/gitea-runner`.
- **Release artifact paths**: S3 directory `act_runner/{{.Version}}` → `gitea-runner/{{.Version}}`, and artifact filenames change with the new project name. Existing download URLs break.
- **Metrics namespace**: changed from `act_runner` to `gitea_runner` (e.g. `act_runner_jobs_total` → `gitea_runner_jobs_total`); existing monitors/dashboards must be updated.
- **ldflags version path**: `gitea.com/gitea/act_runner/internal/pkg/ver.version` → `gitea.com/gitea/runner/internal/pkg/ver.version`. Affects anyone building with custom ldflags.
- **Kubernetes example resource names**: `act-runner` / `act-runner-vol` → `runner` / `runner-vol`. Users who copied the manifests verbatim will see resource churn on apply.
- **s6 service name**: `scripts/s6/act_runner/` → `scripts/s6/gitea-runner/` (image-internal; only matters for downstream image overrides).

Unchanged: YAML config field names, env vars (`GITEA_*`), CLI flags/subcommands, registration file format.
---------

Co-authored-by: silverwind <me@silverwind.io>
Reviewed-on: https://gitea.com/gitea/runner/pulls/850
Reviewed-by: Zettat123 <39446+zettat123@noreply.gitea.com>
Reviewed-by: silverwind <2021+silverwind@noreply.gitea.com>
Reviewed-by: Nicolas <bircni@icloud.com>
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-committed-by: Lunny Xiao <xiaolunwen@gmail.com>

examples/docker-compose/README.md

@@ -1,4 +1,4 @@
-### Running `act_runner` using `docker-compose`
+### Running `gitea-runner` using `docker-compose`
 
 ```yml
 ...
@@ -19,15 +19,15 @@
       # - GITEA_RUNNER_REGISTRATION_TOKEN=<user-defined registration token>
 
   runner:
-    image: gitea/act_runner
+    image: gitea/runner
     restart: always
     depends_on:
       gitea:
         # required so runner can attach to gitea, see "healthcheck"
-        condition: service_healthy 
+        condition: service_healthy
         restart: true
     volumes:
-      - ./data/act_runner:/data
+      - ./data/runner:/data
       - /var/run/docker.sock:/var/run/docker.sock
     environment:
       - GITEA_INSTANCE_URL=<instance url>
@@ -38,18 +38,18 @@
       - GITEA_RUNNER_REGISTRATION_TOKEN=<registration token>
 ```
 
-### Running `act_runner` using Docker-in-Docker (DIND) 
+### Running `gitea-runner` using Docker-in-Docker (DIND)
 
 ```yml
 ...
   runner:
-    image: gitea/act_runner:latest-dind-rootless
+    image: gitea/runner:latest-dind-rootless
     restart: always
     privileged: true
     depends_on:
       - gitea
     volumes:
-      - ./data/act_runner:/data
+      - ./data/runner:/data
     environment:
       - GITEA_INSTANCE_URL=<instance url>
       - DOCKER_HOST=unix:///var/run/user/1000/docker.sock