feat: config command to edit config files (#1140)

Changing a setting after `generate-config` meant hand-editing YAML, which is awkward in provisioning scripts. `config` now edits an existing file in place:

```bash
./gitea-runner -c config.yaml config set runner.capacity 4
./gitea-runner -c config.yaml config set runner.envs.MY_VAR value
./gitea-runner -c config.yaml config add runner.labels 'ubuntu:docker://node:22'
./gitea-runner -c config.yaml config remove runner.labels 'ubuntu:docker://node:22'
./gitea-runner -c config.yaml config get runner.labels
```

Edits go through the YAML node tree, so comments, key order and the blank lines between top-level sections survive — a test asserts that appending a label to `config.example.yaml` changes nothing but the added line. Keys are resolved by reflecting over the `Config` struct's yaml tags, so an unknown key or a value of the wrong type is rejected before the file is touched. The write is atomic and keeps the file's symlink, owner, mode and line endings.

---------

Co-authored-by: silverwind <2021+silverwind@noreply.gitea.com>
Co-authored-by: silverwind <me@silverwind.io>
Reviewed-on: https://gitea.com/gitea/runner/pulls/1140
Reviewed-by: silverwind <2021+silverwind@noreply.gitea.com>
Reviewed-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: bircni <bircni@icloud.com>

internal/app/cmd/config_test.go

@@ -0,0 +1,75 @@
+// Copyright 2026 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package cmd
+
+import (
+	"bytes"
+	"os"
+	"path/filepath"
+	"testing"
+
+	"gitea.com/gitea/runner/internal/pkg/config"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func runConfigCmd(t *testing.T, configFile string, args ...string) (string, string, error) {
+	t.Helper()
+	out, errOut := &bytes.Buffer{}, &bytes.Buffer{}
+	cmd := loadConfigCmd(&configFile)
+	cmd.SetOut(out)
+	cmd.SetErr(errOut)
+	cmd.SetArgs(args)
+	err := cmd.Execute()
+	return out.String(), errOut.String(), err
+}
+
+func TestConfigCmdGeneratePrintsTheExample(t *testing.T) {
+	out, _, err := runConfigCmd(t, "", "generate")
+	require.NoError(t, err)
+	assert.Equal(t, string(config.Example), out)
+}
+
+// The subcommands only wire arguments through, so one pass over all of them is enough.
+func TestConfigCmdEditsTheFile(t *testing.T) {
+	file := filepath.Join(t.TempDir(), "config.yaml")
+	require.NoError(t, os.WriteFile(file, []byte("runner:\n  labels:\n    - self-hosted\n"), 0o600))
+
+	_, _, err := runConfigCmd(t, file, "set", "container.options", "--cpus 2")
+	require.NoError(t, err)
+	_, _, err = runConfigCmd(t, file, "add", "runner.labels", "ubuntu:docker://node:22")
+	require.NoError(t, err)
+	_, _, err = runConfigCmd(t, file, "remove", "runner.labels", "self-hosted")
+	require.NoError(t, err)
+
+	out, _, err := runConfigCmd(t, file, "get", "runner.labels")
+	require.NoError(t, err)
+	assert.Equal(t, "ubuntu:docker://node:22\n", out)
+
+	out, _, err = runConfigCmd(t, file, "get", "container.options")
+	require.NoError(t, err)
+	assert.Equal(t, "--cpus 2\n", out)
+}
+
+func TestConfigCmdResolvesTheConfigFile(t *testing.T) {
+	t.Run("falls back to the working directory", func(t *testing.T) {
+		dir := t.TempDir()
+		require.NoError(t, os.WriteFile(filepath.Join(dir, "config.yaml"), []byte("runner:\n  capacity: 2\n"), 0o600))
+		t.Chdir(dir)
+
+		out, errOut, err := runConfigCmd(t, "", "get", "runner.capacity")
+		require.NoError(t, err)
+		assert.Equal(t, "2\n", out)
+		assert.Contains(t, errOut, "using config file")
+	})
+
+	t.Run("reports that none was found", func(t *testing.T) {
+		t.Chdir(t.TempDir())
+
+		_, _, err := runConfigCmd(t, "", "set", "runner.capacity", "4")
+		require.Error(t, err)
+		assert.Contains(t, err.Error(), "--config")
+	})
+}