feat: add runner health admission checks (#1090)

Opt-in local task-admission checks under a `health_check` config section (disabled by default):

- pause new task fetching when free disk space on the workspace volume is below the configured minimum
- optional executable health-check script — a non-zero exit, timeout, or start failure marks the runner unavailable
- checks run only while the runner is idle; the last result is reused while a job is active, and polling resumes automatically on recovery
- `/readyz` reports task-admission readiness (reusing the poll loop's last check); `/healthz` stays a process-liveness endpoint

---------

Co-authored-by: silverwind <me@silverwind.io>
Reviewed-on: https://gitea.com/gitea/runner/pulls/1090
Reviewed-by: silverwind <2021+silverwind@noreply.gitea.com>

internal/pkg/metrics/metrics_test.go

@@ -81,7 +81,7 @@ func TestRegisterRunningJobsFuncZeroCapacity(t *testing.T) {
 
 func TestStartServerCanBeCancelled(t *testing.T) {
 	ctx, cancel := context.WithCancel(context.Background())
-	StartServer(ctx, "127.0.0.1:0")
+	StartServer(ctx, "127.0.0.1:0", nil)
 	cancel()
 }