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/main_test.go

@@ -684,15 +684,16 @@ func TestRunVerbose(t *testing.T) {
 	}
 
 	checks := []string{
-		"configuration:",
+		"▸ configuration",
 		"git.tag_prefix",
 		"[default]",
-		"branch: release/1.2",
+		"▸ branch",
+		"release/1.2",
 		"major=1, minor=2",
-		"commits analyzed",
+		"▸ commits",
 		"feat: add new thing",
-		"feat → patch bump",
-		"version decision:",
+		"patch bump",
+		"▸ version",
 	}
 	for _, want := range checks {
 		if !strings.Contains(output, want) {