enhance: improve config, comment out values in example file (#1145)

`config.example.yaml` now has every value commented out, as gitea's `app.example.ini` does, so it documents each option with its default instead of imposing it. Copying it no longer pins values the runner would otherwise pick, and a changed default reaches configs that never named the option.

`config init` writes the file to configure: a header comment and no option, so every option keeps its default. It refuses to overwrite an existing config without `--force`, and writes `config.yaml` in the working directory when `-c` is absent.

`config set`, `add` and `remove` keep such a file readable. Comments go back to the indentation they were written at, which the encoder drops for a comment block that has no key below it, and a file of only comments keeps its text instead of being emptied by the first edit.

Also rewrote the config docs sections for clarity.

Reviewed-on: https://gitea.com/gitea/runner/pulls/1145
Reviewed-by: Lunny Xiao <xiaolunwen@gmail.com>
Reviewed-by: bircni <bircni@icloud.com>
Co-authored-by: silverwind <me@silverwind.io>

internal/pkg/config/config_test.go

@@ -348,12 +348,23 @@ cache:
 	assert.Contains(t, err.Error(), "contains no secret")
 }
 
-// The shipped example must parse, and every key in it must be one the config knows.
-func TestLoadDefault_ExampleConfigParses(t *testing.T) {
+// The shipped configs must parse, hold no key the config does not know, and leave
+// every option at its default, as all of their values are commented out.
+func TestLoadDefault_ShippedConfigsChangeNothing(t *testing.T) {
 	hook := test.NewGlobal()
 	defer hook.Reset()
 
-	_, err := LoadDefault("config.example.yaml")
+	defaults, err := LoadDefault("")
 	require.NoError(t, err)
+
+	dir := t.TempDir()
+	for name, content := range map[string][]byte{"config.example.yaml": Example, "minimal.yaml": []byte(Minimal)} {
+		file := filepath.Join(dir, name)
+		require.NoError(t, os.WriteFile(file, content, 0o600))
+
+		cfg, err := LoadDefault(file)
+		require.NoError(t, err, name)
+		assert.Equal(t, defaults, cfg, name)
+	}
 	assert.Empty(t, hook.AllEntries())
 }