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_test.go

@@ -186,6 +186,24 @@ model = "qwen-2.5-coder-32b"
 	}
 }
 
+func TestValidateAutoUpdateRange(t *testing.T) {
+	cfg := DefaultConfig()
+
+	for _, valid := range []int{0, 1, 2} {
+		cfg.Updater.AutoUpdate = valid
+		if err := validate(cfg); err != nil {
+			t.Errorf("expected auto-update=%d to be valid, got error: %v", valid, err)
+		}
+	}
+
+	for _, invalid := range []int{-1, 3} {
+		cfg.Updater.AutoUpdate = invalid
+		if err := validate(cfg); err == nil {
+			t.Errorf("expected auto-update=%d to be rejected", invalid)
+		}
+	}
+}
+
 func TestLoadSave(t *testing.T) {
 	tmpDir, err := os.MkdirTemp("", "iris-config-test")
 	if err != nil {