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>

Dockerfile

@@ -9,8 +9,8 @@ RUN apk add --no-cache make git
 ARG GOPROXY
 ENV GOPROXY=${GOPROXY:-}
 
-COPY . /opt/src/act_runner
-WORKDIR /opt/src/act_runner
+COPY . /opt/src/runner
+WORKDIR /opt/src/runner
 
 RUN make clean && make build
 
@@ -21,7 +21,7 @@ FROM docker:28-dind AS dind
 
 RUN apk add --no-cache s6 bash git tzdata
 
-COPY --from=builder /opt/src/act_runner/act_runner /usr/local/bin/act_runner
+COPY --from=builder /opt/src/runner/gitea-runner /usr/local/bin/gitea-runner
 COPY scripts/run.sh /usr/local/bin/run.sh
 COPY scripts/s6 /etc/s6
 
@@ -37,7 +37,7 @@ FROM docker:28-dind-rootless AS dind-rootless
 USER root
 RUN apk add --no-cache s6 bash git tzdata
 
-COPY --from=builder /opt/src/act_runner/act_runner /usr/local/bin/act_runner
+COPY --from=builder /opt/src/runner/gitea-runner /usr/local/bin/gitea-runner
 COPY scripts/run.sh /usr/local/bin/run.sh
 COPY scripts/s6 /etc/s6
 
@@ -56,7 +56,7 @@ ENTRYPOINT ["s6-svscan","/etc/s6"]
 FROM alpine AS basic
 RUN apk add --no-cache tini bash git tzdata
 
-COPY --from=builder /opt/src/act_runner/act_runner /usr/local/bin/act_runner
+COPY --from=builder /opt/src/runner/gitea-runner /usr/local/bin/gitea-runner
 COPY scripts/run.sh /usr/local/bin/run.sh
 
 VOLUME /data