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/run/disk_windows.go

@@ -0,0 +1,20 @@
+// Copyright 2026 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+//go:build windows
+
+package run
+
+import "golang.org/x/sys/windows"
+
+func freeDiskBytes(path string) (uint64, error) {
+	pathPtr, err := windows.UTF16PtrFromString(path)
+	if err != nil {
+		return 0, err
+	}
+	var available uint64
+	if err := windows.GetDiskFreeSpaceEx(pathPtr, &available, nil, nil); err != nil {
+		return 0, err
+	}
+	return available, nil
+}