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.go

@@ -0,0 +1,116 @@
+// Copyright 2026 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package cmd
+
+import (
+	"fmt"
+	"os"
+	"path/filepath"
+	"slices"
+	"strings"
+
+	"gitea.com/gitea/runner/internal/pkg/config"
+
+	"github.com/spf13/cobra"
+)
+
+func loadConfigCmd(configFile *string) *cobra.Command {
+	configCmd := &cobra.Command{
+		Use:   "config",
+		Short: "Generate, read and edit config files",
+		Args:  cobra.MaximumNArgs(0),
+		RunE: func(cmd *cobra.Command, _ []string) error {
+			return cmd.Help()
+		},
+	}
+
+	configCmd.AddCommand(loadGenerateConfigCmd("generate"))
+
+	configCmd.AddCommand(&cobra.Command{
+		Use:   "get <key>",
+		Short: "Print the value of a config key",
+		Args:  cobra.ExactArgs(1),
+		RunE: func(cmd *cobra.Command, args []string) error {
+			file, err := resolveConfigFile(cmd, configFile)
+			if err != nil {
+				return err
+			}
+			value, err := config.GetValue(file, args[0])
+			if err != nil {
+				return err
+			}
+			fmt.Fprintln(cmd.OutOrStdout(), value)
+			return nil
+		},
+	})
+
+	for _, sub := range []struct {
+		use   string
+		short string
+		edit  func(file, key string, values ...string) error
+	}{
+		{"set <key> <value>...", "Set the value of a config key", config.SetValue},
+		{"add <key> <value>...", "Append values to a list config key", config.AddValue},
+		{"remove <key> <value>...", "Remove values from a list config key", config.RemoveValue},
+	} {
+		valueCmd := &cobra.Command{
+			Use:   sub.use,
+			Short: sub.short,
+			Args:  cobra.MinimumNArgs(2),
+			RunE: func(cmd *cobra.Command, args []string) error {
+				file, err := resolveConfigFile(cmd, configFile)
+				if err != nil {
+					return err
+				}
+				return sub.edit(file, args[0], args[1:]...)
+			},
+		}
+		valueCmd.Flags().SetInterspersed(false) // so a value such as `--cpus 2` is not parsed as a flag
+		configCmd.AddCommand(valueCmd)
+	}
+
+	return configCmd
+}
+
+func loadGenerateConfigCmd(use string) *cobra.Command {
+	return &cobra.Command{
+		Use:   use,
+		Short: "Generate an example config file",
+		Args:  cobra.MaximumNArgs(0),
+		Run: func(cmd *cobra.Command, _ []string) {
+			fmt.Fprintf(cmd.OutOrStdout(), "%s", config.Example)
+		},
+	}
+}
+
+var defaultConfigFileNames = []string{"config.yaml", "config.yml"}
+
+func resolveConfigFile(cmd *cobra.Command, configFile *string) (string, error) {
+	if *configFile != "" {
+		return *configFile, nil
+	}
+
+	var dirs []string
+	if wd, err := os.Getwd(); err == nil {
+		dirs = append(dirs, wd)
+	}
+	if exe, err := os.Executable(); err == nil {
+		if dir := filepath.Dir(exe); !slices.Contains(dirs, dir) {
+			dirs = append(dirs, dir)
+		}
+	}
+
+	for _, dir := range dirs {
+		for _, name := range defaultConfigFileNames {
+			candidate := filepath.Join(dir, name)
+			if stat, err := os.Stat(candidate); err == nil && !stat.IsDir() {
+				fmt.Fprintf(cmd.ErrOrStderr(), "using config file %q\n", candidate)
+				return candidate, nil
+			}
+		}
+	}
+
+	return "", fmt.Errorf("no %s found in %s, pass one with --config",
+		strings.Join(defaultConfigFileNames, " or "), strings.Join(dirs, " or "))
+}