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/app/cmd/daemon.go

@@ -147,17 +147,19 @@ func runDaemon(ctx context.Context, daemArgs *daemonArgs, configFile *string) fu
 		}
 		runner.SetCapabilitiesFromDeclare(resp)
 
+		poller := poll.New(cfg, cli, runner)
+
 		if cfg.Metrics.Enabled {
 			metrics.Init()
 			metrics.RunnerInfo.WithLabelValues(ver.Version(), resp.Msg.Runner.Name).Set(1)
 			metrics.RunnerCapacity.Set(float64(cfg.Runner.Capacity))
 			metrics.RegisterUptimeFunc(time.Now())
 			metrics.RegisterRunningJobsFunc(runner.RunningCount, cfg.Runner.Capacity)
-			metrics.StartServer(ctx, cfg.Metrics.Addr)
+			metrics.StartServer(ctx, cfg.Metrics.Addr, func() (bool, string) {
+				return poller.Ready(cfg.Metrics.ReadinessGrace)
+			})
 		}
 
-		poller := poll.New(cfg, cli, runner)
-
 		if daemArgs.Once || reg.Ephemeral {
 			done := make(chan struct{})
 			go func() {