feat: update changelog (#131)

closes #130 

- CHANGELOG.md file for people who can view changelog from github
---
`iris changelog` command:
`iris changelog [version]` (e.g. iris changelog v0.5.1)
`iris changelog -n 3` (see changelog of 3 latest releaseas)

=> `iris changelog` looks just like CHANGELOG.md, grouped headers and
all. It only stops looking that way if we personally overwrite that
release's description with something else
=> With 1-hour local cache (invalidated on updater-channel change),
`--refresh` flag to bypass, graceful fallback to stale cache on GitHub
rate-limiting

---
- New auto update config
```toml
# 0 = off (default, notify only), 1 = auto-install, 2 = always confirm first
auto-update = 1
```

- New just debug-changelog and just debug-autoupdate recipes (fully
offline, mocked HTTP servers, no real network/config touched)

internal/config/config.go

@@ -61,6 +61,8 @@ type UpdaterConfig struct {
 	CheckOnStartup bool     `toml:"check-on-startup"`
 	Channel        string   `toml:"channel"`
 	CheckInterval  Duration `toml:"check-interval"`
+	// AutoUpdate: 0 = off (default), 1 = auto-install, 2 = always confirm first
+	AutoUpdate int `toml:"auto-update"`
 }
 
 type KeybindingsConfig struct {
@@ -281,6 +283,10 @@ func validate(cfg *Config) error {
 		return fmt.Errorf("updater.channel: invalid value %q (want: stable|nightly)", cfg.Updater.Channel)
 	}
 
+	if cfg.Updater.AutoUpdate < 0 || cfg.Updater.AutoUpdate > 2 {
+		return fmt.Errorf("updater.auto-update: invalid value %d (want: 0=off, 1=auto, 2=confirm)", cfg.Updater.AutoUpdate)
+	}
+
 	if cfg.UI.MaxSuggestions < 1 || cfg.UI.MaxSuggestions > 500 {
 		return fmt.Errorf("ui.max-suggestions: must be between 1 and 500")
 	}