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/app/cmd/config_test.go

@@ -32,6 +32,30 @@ func TestConfigCmdGeneratePrintsTheExample(t *testing.T) {
 	assert.Equal(t, string(config.Example), out)
 }
 
+func TestConfigCmdInitWritesTheMinimalConfig(t *testing.T) {
+	dir := t.TempDir()
+	file := filepath.Join(dir, "config.yaml")
+
+	out, _, err := runConfigCmd(t, file, "init")
+	require.NoError(t, err)
+	assert.Contains(t, out, file)
+	content, err := os.ReadFile(file)
+	require.NoError(t, err)
+	assert.Equal(t, config.Minimal, string(content))
+
+	_, _, err = runConfigCmd(t, file, "init")
+	require.Error(t, err)
+	assert.Contains(t, err.Error(), "--force")
+
+	_, _, err = runConfigCmd(t, file, "init", "--force")
+	require.NoError(t, err)
+
+	t.Chdir(t.TempDir())
+	_, _, err = runConfigCmd(t, "", "init")
+	require.NoError(t, err)
+	assert.FileExists(t, defaultConfigFileNames[0])
+}
+
 // 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")