fix(builder): 版本时间格式改回 YYYYMMDDTHHMM

按要求保留字母分隔符：20260918T1722。分钟精度不变，同一分支同日多次
构建仍能正确排序。

记录切换边界：Debian 里字母排在标点之前，YYYYMMDDT… 小于旧格式的
YYYYMMDD.…，切换当天同日的旧格式包会压住新包，需要在仓库里把那些旧包
移除或停用；次日新格式始终更大。回归测试显式断言这行边界。

skills/builder/scripts/version.sh

@@ -11,19 +11,20 @@ With --docker, print the Docker tag rendering of that version.
 
 Derivation (builder contract «版本号»):
   official  HEAD exact-match of vX.Y.Z          → X.Y.Z
-  test      nearest ancestor stable tag vX.Y.Z  → X.Y.(Z+1)~YYYYMMDDHHMM.branch.n+gSHA
-  no tag    no stable tag reachable from HEAD   → 0.0.0~YYYYMMDDHHMM.branch+gSHA
+  test      nearest ancestor stable tag vX.Y.Z  → X.Y.(Z+1)~YYYYMMDDTHHMM.branch.n+gSHA
+  no tag    no stable tag reachable from HEAD   → 0.0.0~YYYYMMDDTHHMM.branch+gSHA
 
-YYYYMMDDHHMM is the build minute (day plus time of day), with no separator:
-the digits must stay in one run so Debian compares them numerically. A letter
-separator (YYYYMMDDTHHMM) would compare *below* the bare date, because Debian
-orders letters before punctuation — same-day packages built with the old
-date-only scheme would outrank the new ones. SOURCE_DATE_EPOCH, if set, is
-that instant in UTC; otherwise the local date and time. Official X.Y.Z never
-gets a date. Minute precision is required for apt: Debian version comparison
-ignores upload order, so same-day rebuilds that differ only in +gSHA compare
-as arbitrary text (a newer build can sort below an older one). With the minute
-in the version, later builds always compare greater.
+YYYYMMDDTHHMM is the build minute (day plus time of day). SOURCE_DATE_EPOCH,
+if set, is that instant in UTC; otherwise the local date and time. Official
+X.Y.Z never gets a date. Minute precision is required for apt: Debian version
+comparison ignores upload order, so same-day rebuilds that differ only in
++gSHA compare as arbitrary text (a newer build can sort below an older one).
+
+Switching from the date-only scheme: Debian orders letters below punctuation,
+so 'YYYYMMDDT…' compares *below* the old 'YYYYMMDD.…' form. Packages built on
+the switch day therefore rank under that day's date-only packages until those
+are removed or disabled in the repository; from the next day on, the new form
+always ranks higher.
 
 Stable tags match v<major>.<minor>.<patch> only. Baseline is ancestry, not
 the highest version in the repository. This script does not fetch tags.
@@ -120,17 +121,15 @@ short_sha() {
   gitc rev-parse HEAD | cut -c1-7
 }
 
-# Build-minute YYYYMMDDHHMM. SOURCE_DATE_EPOCH (UTC) wins when set, for
+# Build-minute YYYYMMDDTHHMM. SOURCE_DATE_EPOCH (UTC) wins when set, for
 # reproducible test packages; otherwise the machine's current local time.
 # The minute is not decoration: it is what keeps same-day rebuilds ordered in
-# Debian version comparison (0.0.0~202609181036… < 0.0.0~202609181722…). It is
-# glued to the date with no 'T' so the whole stamp stays one numeric run;
-# '20260918T1722' would sort below '20260918.dev-…' and lose to old packages.
+# Debian version comparison (0.0.0~20260918T1036… < 0.0.0~20260918T1722…).
 build_stamp() {
   if [[ -n "${SOURCE_DATE_EPOCH:-}" ]]; then
-    date -u -d "@${SOURCE_DATE_EPOCH}" +%Y%m%d%H%M
+    date -u -d "@${SOURCE_DATE_EPOCH}" +%Y%m%dT%H%M
   else
-    date +%Y%m%d%H%M
+    date +%Y%m%dT%H%M
   fi
 }