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>

Makefile

@@ -1,5 +1,5 @@
 DIST := dist
-EXECUTABLE := act_runner
+EXECUTABLE := gitea-runner
 DIST_DIRS := $(DIST)/binaries $(DIST)/release
 GO ?= go
 SHASUM ?= shasum -a 256
@@ -13,7 +13,7 @@ DARWIN_ARCHS ?= darwin-12/amd64,darwin-12/arm64
 WINDOWS_ARCHS ?= windows/amd64
 GOFILES := $(shell find . -type f -name "*.go" -o -name "go.mod" ! -name "generated.*")
 
-DOCKER_IMAGE ?= gitea/act_runner
+DOCKER_IMAGE ?= gitea/runner
 DOCKER_TAG ?= nightly
 DOCKER_REF := $(DOCKER_IMAGE):$(DOCKER_TAG)
 DOCKER_ROOTLESS_REF := $(DOCKER_IMAGE):$(DOCKER_TAG)-dind-rootless
@@ -67,7 +67,7 @@ else
 endif
 
 TAGS ?=
-LDFLAGS ?= -X "gitea.com/gitea/act_runner/internal/pkg/ver.version=v$(RELASE_VERSION)"
+LDFLAGS ?= -X "gitea.com/gitea/runner/internal/pkg/ver.version=v$(RELASE_VERSION)"
 
 .PHONY: all
 all: build
@@ -86,7 +86,7 @@ go-check:
 	$(eval MIN_GO_VERSION := $(shell printf "%03d%03d" $(shell echo '$(MIN_GO_VERSION_STR)' | tr '.' ' ')))
 	$(eval GO_VERSION := $(shell printf "%03d%03d" $(shell $(GO) version | grep -Eo '[0-9]+\.[0-9]+' | tr '.' ' ');))
 	@if [ "$(GO_VERSION)" -lt "$(MIN_GO_VERSION)" ]; then \
-		echo "Act Runner requires Go $(MIN_GO_VERSION_STR) or greater to build. You can get it at https://go.dev/dl/"; \
+		echo "Gitea Runner requires Go $(MIN_GO_VERSION_STR) or greater to build. You can get it at https://go.dev/dl/"; \
 		exit 1; \
 	fi
 
@@ -140,11 +140,11 @@ test: fmt-check security-check ## test everything
 	@$(GO) test -race -short -v -cover -coverprofile coverage.txt ./... && echo "\n==>\033[32m Ok\033[m\n" || exit 1
 
 .PHONY: install
-install: $(GOFILES) ## install the act_runner binary via `go install`
+install: $(GOFILES) ## install the runner binary via `go install`
 	$(GO) install -v -tags '$(TAGS)' -ldflags '-s -w $(EXTLDFLAGS) $(LDFLAGS)'
 
 .PHONY: build
-build: go-check $(EXECUTABLE) ## build the act_runner binary
+build: go-check $(EXECUTABLE) ## build the runner binary
 
 $(EXECUTABLE): $(GOFILES)
 	$(GO) build -v -tags '$(TAGS)' -ldflags '-s -w $(EXTLDFLAGS) $(LDFLAGS)' -o $@