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.go

@@ -24,6 +24,12 @@ import (
 // (so a programmatically built config still gets a sane bound).
 const DefaultPostTaskScriptTimeout = 5 * time.Minute
 
+// Minimal is the smallest config file that runs the runner: options it does not
+// name keep their default, and it names none.
+const Minimal = `# Minimal config file. Every option it does not set keeps its default.
+# "gitea-runner config generate" prints all options, "config set <key> <value>" sets one here.
+`
+
 // Log represents the configuration for logging.
 type Log struct {
 	Level string `yaml:"level"` // Level indicates the logging level.