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_test.go

@@ -0,0 +1,53 @@
+// Copyright 2026 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package run
+
+import (
+	"testing"
+
+	"gitea.com/gitea/runner/internal/pkg/config"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestCanAcceptTaskDiskGuard(t *testing.T) {
+	cfg, err := config.LoadDefault("")
+	require.NoError(t, err)
+	cfg.Host.WorkdirParent = t.TempDir()
+	cfg.HealthCheck.Enabled = true
+	r := &Runner{cfg: cfg}
+
+	ready, _ := r.CanAcceptTask(t.Context())
+	assert.True(t, ready)
+
+	cfg.HealthCheck.MinFreeDiskSpaceMB = 1 << 40
+	ready, reason := r.CanAcceptTask(t.Context())
+	assert.False(t, ready)
+	assert.Contains(t, reason, "low disk space")
+}
+
+func TestDiskCheckDeferredWhileJobRuns(t *testing.T) {
+	cfg, err := config.LoadDefault("")
+	require.NoError(t, err)
+	cfg.HealthCheck.Enabled = true
+	cfg.HealthCheck.MinFreeDiskSpaceMB = 1
+	cfg.Host.WorkdirParent = t.TempDir()
+	r := &Runner{cfg: cfg}
+
+	ready, reason := r.CanAcceptTask(t.Context())
+	assert.True(t, ready)
+	assert.Equal(t, "ok", reason)
+
+	cfg.HealthCheck.MinFreeDiskSpaceMB = 1 << 40
+	r.runningCount.Store(1)
+	ready, reason = r.CanAcceptTask(t.Context())
+	assert.True(t, ready, "the disk check must not run while a job is active")
+	assert.Equal(t, "ok", reason)
+
+	r.runningCount.Store(0)
+	ready, reason = r.CanAcceptTask(t.Context())
+	assert.False(t, ready)
+	assert.Contains(t, reason, "low disk space")
+}