feat: git retime 改为在 --start-date/--end-date 区间内均匀分布提交

- 新增 --end-date：提交按比例铺满 [start-date, end-date]，第 i 个提交
  落在第 i×天数÷提交数 天，提交远少于天数时也不会堆在第一天
- 移除 --per-day / --day-gap：原先的“每天一个、日期顺延”语义不直观
- 只给 --start-date 时集中在该日；--end-date 需配合 --start-date 且不早于它
- 同一天的多条提交仍按顺序落在 21:00-23:00 之间
- 文档与预览文案同步为区间语义

internal/executor/gitretime_cmd.go

@@ -29,8 +29,7 @@ type gitRetimeOptions struct {
 	// Redating: when StartDate is set, the selected commits are re-dated in
 	// order starting there instead of keeping their original days.
 	StartDate time.Time
-	DayGap    int
-	PerDay    int
+	EndDate   time.Time // defaults to StartDate when only the start is given
 }
 
 // runGitRetime rewrites the timestamps of commits in [From, To] so they land
@@ -118,13 +117,12 @@ func writeRetimePreview(req BuiltinRequest, plan retimePlan, opts gitRetimeOptio
 	fmt.Fprintf(w, "目标:   %s\n", target)
 	dateRule := "保留原日期"
 	if !opts.StartDate.IsZero() {
-		dateRule = "从 " + opts.StartDate.Format("2006-01-02") + " 起重排"
-		if opts.PerDay > 0 {
-			dateRule += fmt.Sprintf("，每天 %d 个", opts.PerDay)
-		}
-		if opts.DayGap > 1 {
-			dateRule += fmt.Sprintf("，间隔 %d 天", opts.DayGap)
+		end := opts.EndDate
+		if end.IsZero() {
+			end = opts.StartDate
 		}
+		dateRule = fmt.Sprintf("在 %s 至 %s 之间均匀分布",
+			opts.StartDate.Format("2006-01-02"), end.Format("2006-01-02"))
 	}
 	fmt.Fprintf(w, "窗口:   %s-%s（%s）\n\n",
 		clockOf(opts.Window), clockOf(opts.WindowEnd), dateRule)
@@ -218,8 +216,7 @@ func collectRetimePlan(ctx context.Context, runner gitRunner, opts gitRetimeOpti
 		FirstGapMax: 20 * time.Minute,
 		Rand:        newRetimeRand(opts.Seed),
 		StartDate:   opts.StartDate,
-		DayGap:      opts.DayGap,
-		PerDay:      opts.PerDay,
+		EndDate:     opts.EndDate,
 	}
 	_ = opts.MatchEmote
 	return buildPlan(selected, plannerOpts, branch, remote), nil
@@ -388,26 +385,14 @@ func parseGitRetimeArgs(args []string, now time.Time) (gitRetimeOptions, bool, e
 			if opts.StartDate, err = parseDate(v); err != nil {
 				return opts, false, err
 			}
-		case "--day-gap":
+		case "--end-date":
 			v, err := takeValue()
 			if err != nil {
 				return opts, false, err
 			}
-			n, err := strconv.Atoi(v)
-			if err != nil || n < 1 {
-				return opts, false, fmt.Errorf("--day-gap 需要大于 0 的整数（天数），收到 %q", v)
-			}
-			opts.DayGap = n
-		case "--per-day":
-			v, err := takeValue()
-			if err != nil {
+			if opts.EndDate, err = parseDate(v); err != nil {
 				return opts, false, err
 			}
-			n, err := strconv.Atoi(v)
-			if err != nil || n < 1 {
-				return opts, false, fmt.Errorf("--per-day 需要大于 0 的整数（每天提交数），收到 %q", v)
-			}
-			opts.PerDay = n
 		default:
 			return opts, false, fmt.Errorf("未知选项 %q", arg)
 		}
@@ -437,16 +422,16 @@ func parseGitRetimeArgs(args []string, now time.Time) (gitRetimeOptions, bool, e
 	if opts.MaxGap <= opts.MinGap {
 		return opts, false, fmt.Errorf("间隔上限必须大于下限")
 	}
-	// --day-gap / --per-day only make sense together with --start-date.
+	// --end-date only makes sense together with --start-date, and must not
+	// end before it starts.
 	if opts.StartDate.IsZero() {
-		if opts.DayGap > 0 || opts.PerDay > 0 {
-			return opts, false, fmt.Errorf("--day-gap 与 --per-day 需要配合 --start-date 使用" +
+		if !opts.EndDate.IsZero() {
+			return opts, false, fmt.Errorf("--end-date 需要配合 --start-date 使用" +
 				"（不给 --start-date 时保留每个提交的原日期）")
 		}
-		opts.DayGap = 1
-	} else if opts.DayGap < 1 {
-		// Re-dating defaults to consecutive days.
-		opts.DayGap = 1
+	} else if !opts.EndDate.IsZero() && opts.EndDate.Before(opts.StartDate) {
+		return opts, false, fmt.Errorf("--end-date %s 早于 --start-date %s",
+			opts.EndDate.Format("2006-01-02"), opts.StartDate.Format("2006-01-02"))
 	}
 	return opts, confirmed, nil
 }