feat(actions): add artifact listing and download (#210)

Adds the ability to list and download Gitea Actions artifacts. Extends the existing `actions_run_read` tool with four new methods rather than adding a separate tool, keeping artifacts alongside the runs/jobs/logs they belong to.

### New methods (`actions_run_read`)

| Method               | Description                                | Key params                                             |
| -------------------- | ------------------------------------------ | ------------------------------------------------------ |
| `list_artifacts`     | List all artifacts in a repository         | `owner`, `repo`, optional `artifact_name` filter       |
| `list_run_artifacts` | List artifacts for one workflow run        | `owner`, `repo`, `run_id`                              |
| `get_artifact`       | Get metadata for a single artifact         | `owner`, `repo`, `artifact_id`                         |
| `download_artifact`  | Download an artifact's zip archive to disk | `owner`, `repo`, `artifact_id`, optional `output_path` |Reviewed-on: https://gitea.com/gitea/gitea-mcp/pulls/210
Reviewed-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: bircni <bircni@icloud.com>

operation/actions/slim.go

@@ -90,3 +90,19 @@ func slimActionWorkflow(raw any) any {
 func slimActionWorkflows(raw any) any {
 	return slimPaginated(raw, slimWorkflow)
 }
+
+func slimArtifact(m map[string]any) map[string]any {
+	return pick(m, "id", "name", "size_in_bytes", "expired",
+		"created_at", "updated_at", "expires_at")
+}
+
+func slimActionArtifact(raw any) any {
+	if m, ok := raw.(map[string]any); ok {
+		return slimArtifact(m)
+	}
+	return raw
+}
+
+func slimActionArtifacts(raw any) any {
+	return slimPaginated(raw, slimArtifact)
+}