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>

internal/pkg/config/config.go

@@ -48,7 +48,7 @@ type Cache struct {
 	Host           string `yaml:"host"`            // Host specifies the caching host.
 	Port           uint16 `yaml:"port"`            // Port specifies the caching port.
 	ExternalServer string `yaml:"external_server"` // ExternalServer specifies the URL of external cache server
-	ExternalSecret string `yaml:"external_secret"` // ExternalSecret is a shared secret between this runner and an external act_runner cache-server, enabling per-job ACTIONS_RUNTIME_TOKEN authentication and repo scoping over the network. Leave empty to keep the legacy unauthenticated behavior.
+	ExternalSecret string `yaml:"external_secret"` // ExternalSecret is a shared secret between this runner and an external gitea-runner cache-server, enabling per-job ACTIONS_RUNTIME_TOKEN authentication and repo scoping over the network. Leave empty to keep the legacy unauthenticated behavior.
 }
 
 // Container represents the configuration for the container.
@@ -62,8 +62,8 @@ type Container struct {
 	DockerHost    string        `yaml:"docker_host"`    // DockerHost specifies the Docker host. It overrides the value specified in environment variable DOCKER_HOST.
 	ForcePull     bool          `yaml:"force_pull"`     // Pull docker image(s) even if already present
 	ForceRebuild  bool          `yaml:"force_rebuild"`  // Rebuild docker image(s) even if already present
-	RequireDocker bool          `yaml:"require_docker"` // Always require a reachable docker daemon, even if not required by act_runner
-	DockerTimeout time.Duration `yaml:"docker_timeout"` // Timeout to wait for the docker daemon to be reachable, if docker is required by require_docker or act_runner
+	RequireDocker bool          `yaml:"require_docker"` // Always require a reachable docker daemon, even if not required by runner
+	DockerTimeout time.Duration `yaml:"docker_timeout"` // Timeout to wait for the docker daemon to be reachable, if docker is required by require_docker or runner
 	BindWorkdir   bool          `yaml:"bind_workdir"`   // BindWorkdir binds the workspace to the host filesystem instead of using Docker volumes. Required for DinD when jobs use docker compose with bind mounts.
 }
 
@@ -138,7 +138,7 @@ func LoadDefault(file string) (*Config, error) {
 			cfg.Cache.Dir = filepath.Join(home, ".cache", "actcache")
 		}
 		if cfg.Cache.ExternalServer != "" && cfg.Cache.ExternalSecret == "" {
-			return nil, errors.New("cache.external_server is set but cache.external_secret is empty; configure the same external_secret on this runner and the act_runner cache-server")
+			return nil, errors.New("cache.external_server is set but cache.external_secret is empty; configure the same external_secret on this runner and the gitea-runner cache-server")
 		}
 	}
 	if cfg.Container.WorkdirParent == "" {
@@ -190,7 +190,7 @@ func LoadDefault(file string) (*Config, error) {
 		if cfg.Container.NetworkMode == "bridge" {
 			// Previously, if the value of `container.network_mode` is `bridge`, we will create a new network for job.
 			// But “bridge” is easily confused with the bridge network created by Docker by default.
-			// So we set the value of `container.network` to empty string to make `act_runner` automatically create a new network for job.
+			// So we set the value of `container.network` to empty string to make `runner` automatically create a new network for job.
 			cfg.Container.Network = ""
 		} else {
 			cfg.Container.Network = cfg.Container.NetworkMode