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/pkg/config/edit_other.go

@@ -0,0 +1,13 @@
+// Copyright 2026 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+//go:build windows || plan9
+
+package config
+
+import "os"
+
+// preserveOwner is a no-op where a new file inherits its ownership from the directory.
+func preserveOwner(_ string, _ os.FileInfo) error {
+	return nil
+}