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/state.go

@@ -11,6 +11,10 @@ import (
 type UpdaterState struct {
 	LastCheckTime time.Time `toml:"last-check-time"`
 	SeenVersion   string    `toml:"seen-version"`
+	// auto-update escalation ladder state, see decideAutoUpdateAction
+	AutoUpdateTarget  string `toml:"auto-update-target"`
+	AutoUpdateAttempt int    `toml:"auto-update-attempt"`
+	DeclinedVersion   string `toml:"declined-version"`
 }
 
 type State struct {