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/defaults.go

@@ -32,6 +32,7 @@ func DefaultConfig() *Config {
 			CheckOnStartup: true,
 			Channel:        "stable",
 			CheckInterval:  Duration(24 * time.Hour),
+			AutoUpdate:     0,
 		},
 		AI: AIConfig{
 			Enabled:       false,
@@ -59,8 +60,11 @@ func DefaultState() *State {
 	return &State{
 		LastMode: "spec",
 		Updater: UpdaterState{
-			LastCheckTime: time.Time{},
-			SeenVersion:   "",
+			LastCheckTime:     time.Time{},
+			SeenVersion:       "",
+			AutoUpdateTarget:  "",
+			AutoUpdateAttempt: 0,
+			DeclinedVersion:   "",
 		},
 	}
 }