feat: git retime 支持重排提交日期，新增 install.sh

- 新增 --start-date：按原顺序从指定日期起重排提交日期，默认每天一个
- 新增 --per-day 控制每天提交数、--day-gap 控制日期间隔；两者需配合
  --start-date 使用，否则报错提示
- 不给 --start-date 时保持原行为（保留原日期，只改时分秒）
- 预览在重排模式下显示完整的新日期时间，并说明日期规则
- install.sh：补 shebang 与可执行权限，构建后安装到 /usr/local/bin
- 文档：README.git.md 增加「重排日期」小节与选项表

internal/executor/gitretime_cmd_test.go

@@ -423,3 +423,47 @@ func fixtureResp(args ...string) gitResponse {
 	}
 	return r
 }
+
+func TestParseGitRetimeArgsRedate(t *testing.T) {
+	opts, _, err := parseGitRetimeArgs([]string{
+		"2026-10-01", "2026-10-15",
+		"--start-date", "2026-11-01",
+		"--per-day", "2",
+		"--day-gap", "3",
+	}, time.Now())
+	if err != nil {
+		t.Fatalf("parse: %v", err)
+	}
+	if opts.StartDate.Format("2006-01-02") != "2026-11-01" || opts.PerDay != 2 || opts.DayGap != 3 {
+		t.Fatalf("opts = %+v", opts)
+	}
+}
+
+func TestParseGitRetimeArgsRedateValidation(t *testing.T) {
+	tests := []struct {
+		args []string
+		want string
+	}{
+		{[]string{"2026-10-01", "2026-10-02", "--per-day", "2"}, "需要配合 --start-date"},
+		{[]string{"2026-10-01", "2026-10-02", "--day-gap", "2"}, "需要配合 --start-date"},
+		{[]string{"2026-10-01", "2026-10-02", "--start-date", "2026/11/01"}, "YYYY-MM-DD"},
+		{[]string{"2026-10-01", "2026-10-02", "--start-date", "2026-11-01", "--per-day", "0"}, "大于 0"},
+		{[]string{"2026-10-01", "2026-10-02", "--start-date", "2026-11-01", "--day-gap", "x"}, "大于 0"},
+	}
+	for _, tc := range tests {
+		_, _, err := parseGitRetimeArgs(tc.args, time.Now())
+		if err == nil || !strings.Contains(err.Error(), tc.want) {
+			t.Errorf("args %v: err = %v, want %q", tc.args, err, tc.want)
+		}
+	}
+}
+
+func TestParseGitRetimeArgsDefaultDayGap(t *testing.T) {
+	opts, _, err := parseGitRetimeArgs([]string{"2026-10-01", "2026-10-02", "--start-date", "2026-11-01"}, time.Now())
+	if err != nil {
+		t.Fatalf("parse: %v", err)
+	}
+	if opts.DayGap != 1 {
+		t.Errorf("DayGap = %d, want the default 1", opts.DayGap)
+	}
+}