feat(ui): add colored, structured CLI output

Replace flat "info:" / "warning:" stderr lines with:
- logStep (·), logDone (✓), logWarn (!) prefix symbols
- ANSI colors when stderr is a TTY; auto-disabled via NO_COLOR or TERM=dumb
- Verbose mode uses ▸ section headers (configuration / branch / commits / version)
- Config table source tags colored: dim=[default], bold=[config file],
  cyan=[env:], green=[flag:]
- Commit table in verbose mode: type column colored by kind (cyan=feat,
  green=fix, red=breaking), ignored commits dimmed
- New cmd/ui.go holds all color helpers (paint, logStep, logDone, logWarn,
  logSection, fmtSource); removes the vlog() helper

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

cmd/ui.go

@@ -0,0 +1,69 @@
+package main
+
+import (
+	"fmt"
+	"os"
+	"strings"
+)
+
+const (
+	ansiReset  = "\033[0m"
+	ansiBold   = "\033[1m"
+	ansiDim    = "\033[2m"
+	ansiRed    = "\033[31m"
+	ansiGreen  = "\033[32m"
+	ansiYellow = "\033[33m"
+	ansiCyan   = "\033[36m"
+)
+
+var useColor bool
+
+func init() {
+	fi, err := os.Stderr.Stat()
+	tty := err == nil && (fi.Mode()&os.ModeCharDevice) != 0
+	useColor = tty && os.Getenv("NO_COLOR") == "" && os.Getenv("TERM") != "dumb"
+}
+
+func paint(code, s string) string {
+	if !useColor {
+		return s
+	}
+	return code + s + ansiReset
+}
+
+// logStep writes a neutral progress line to stderr.
+func logStep(format string, args ...any) {
+	msg := fmt.Sprintf(format, args...)
+	fmt.Fprintf(os.Stderr, "  %s %s\n", paint(ansiDim, "·"), msg)
+}
+
+// logDone writes a success completion line to stderr.
+func logDone(format string, args ...any) {
+	msg := fmt.Sprintf(format, args...)
+	fmt.Fprintf(os.Stderr, "  %s %s\n", paint(ansiGreen+ansiBold, "✓"), msg)
+}
+
+// logWarn writes a warning line to stderr.
+func logWarn(format string, args ...any) {
+	msg := fmt.Sprintf(format, args...)
+	fmt.Fprintf(os.Stderr, "  %s %s\n", paint(ansiYellow, "!"), msg)
+}
+
+// logSection writes a bold section header to stderr (used in verbose mode).
+func logSection(title string) {
+	fmt.Fprintf(os.Stderr, "\n%s\n", paint(ansiBold, "▸ "+title))
+}
+
+// fmtSource returns a colored "[source]" tag for a config key source.
+func fmtSource(src string) string {
+	switch {
+	case strings.HasPrefix(src, "env:"):
+		return paint(ansiCyan, "["+src+"]")
+	case strings.HasPrefix(src, "flag:"):
+		return paint(ansiGreen, "["+src+"]")
+	case src == "default":
+		return paint(ansiDim, "[default]")
+	default: // "config file"
+		return paint(ansiBold, "["+src+"]")
+	}
+}