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)

root/update_test.go

@@ -1,8 +1,10 @@
 package root
 
 import (
+	"io"
 	"os"
 	"path/filepath"
+	"strings"
 	"testing"
 	"time"
 
@@ -76,3 +78,78 @@ func TestUpdateState(t *testing.T) {
 		t.Errorf("Expected LastCheck %v, got %v", state.Updater.LastCheckTime, loaded.Updater.LastCheckTime)
 	}
 }
+
+func TestChangelogSummaryLines(t *testing.T) {
+	body := "## Changelog\n### Bug fixes\n* abc1234  fix something\n* def5678  fix another thing\n* ghi9012  a third fix\n## Update\n```bash\niris update\n```\n"
+
+	got := changelogSummaryLines(body, 2)
+	want := []string{"fix something", "fix another thing"}
+	if len(got) != len(want) {
+		t.Fatalf("expected %d lines, got %d: %v", len(want), len(got), got)
+	}
+	for i, line := range want {
+		if got[i] != line {
+			t.Errorf("line %d: expected %q, got %q", i, line, got[i])
+		}
+	}
+}
+
+func TestChangelogSummaryLinesEmptyBody(t *testing.T) {
+	if got := changelogSummaryLines("", 2); len(got) != 0 {
+		t.Errorf("expected no lines for an empty body, got %v", got)
+	}
+}
+
+func TestPrintUpdateNoticeIncludesChangelogSummary(t *testing.T) {
+	originalVersion := Version
+	Version = "v1.0.0"
+	t.Cleanup(func() { Version = originalVersion })
+
+	r, w, err := os.Pipe()
+	if err != nil {
+		t.Fatalf("os.Pipe: %v", err)
+	}
+	originalStdout := os.Stdout
+	os.Stdout = w
+
+	printUpdateNotice("v1.1.0", "## Changelog\n### Bug fixes\n* abc1234  fix something\n* def5678  fix another thing\n* ghi9012  a third fix\n")
+
+	_ = w.Close()
+	os.Stdout = originalStdout
+	out, _ := io.ReadAll(r)
+	got := string(out)
+
+	if !strings.Contains(got, "v1.0.0") || !strings.Contains(got, "v1.1.0") {
+		t.Errorf("expected the version transition in the notice, got %q", got)
+	}
+	if !strings.Contains(got, "fix something") || !strings.Contains(got, "fix another thing") {
+		t.Errorf("expected the first two changelog entries in the notice, got %q", got)
+	}
+	if strings.Contains(got, "a third fix") {
+		t.Errorf("expected the summary to cap at 2 entries, got %q", got)
+	}
+}
+
+func TestPrintUpdateNoticeWithoutNotes(t *testing.T) {
+	originalVersion := Version
+	Version = "v1.0.0"
+	t.Cleanup(func() { Version = originalVersion })
+
+	r, w, err := os.Pipe()
+	if err != nil {
+		t.Fatalf("os.Pipe: %v", err)
+	}
+	originalStdout := os.Stdout
+	os.Stdout = w
+
+	printUpdateNotice("v1.1.0", "")
+
+	_ = w.Close()
+	os.Stdout = originalStdout
+	out, _ := io.ReadAll(r)
+	got := string(out)
+
+	if !strings.Contains(got, "v1.1.0") {
+		t.Errorf("expected the notice to still show without changelog notes, got %q", got)
+	}
+}