fix(#1871): wire phase archival end-to-end (phases archive cmd + default + atomic) (#1924)

Follow-up to #1919 (archive-then-remove core). Closes the remaining #1871
acceptance criteria so phase history is preserved across the full milestone
lifecycle, not just at phases.clear:

- #2 src/milestone.cts + src/phases-command-router.cts: extract shared
  archivePhaseDirectories() helper; add cmdPhasesArchive (the previously
  half-wired phases.archive alias now routes instead of erroring Unknown).
- #4 gsd-tools.cjs + src/milestone.cts: milestone complete archives phase
  dirs by default (--no-archive-phases opts out; --archive-phases is now a
  harmless no-op). complete-milestone.md updated to drop the redundant manual
  Yes/Skip archive prompt.
- #3 gsd-core/workflows/new-milestone.md: §6 stages the archive move + source
  removal (git add .planning/milestones/ .planning/phases/) in the same commit
  as the milestone start, so the archive lands atomically — no orphaned
  uncommitted deletions, no un-archived dirs inherited.
- docs/CLI-TOOLS.md (+ ja/zh/ko/pt) + help/modes/full.md: flag accuracy.
- tests: phases archive command (#2) + milestone complete default archive /
  --no-archive-phases opt-out (#4). Goldens + workflow size baseline refreshed.

Closes #1871

.changeset/plucky-bears-snooze.md

@@ -0,0 +1,5 @@
+---
+type: Fixed
+pr: 1924
+---
+**Phase archival is now wired end-to-end across the milestone lifecycle** — finishes the #1871 follow-up: `phases archive` is now a real command (the half-wired alias is routed, no longer errors Unknown), `milestone complete` archives phase dirs by default (`--no-archive-phases` opts out), and `new-milestone` §6 stages the archive move + source removal in the same commit so history is preserved atomically rather than left as orphaned uncommitted deletions. (#1871)