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

tests/workflow-size-baseline.json

@@ -13,7 +13,7 @@
   "cleanup.md": 9897,
   "code-review-fix.md": 23890,
   "code-review.md": 31602,
-  "complete-milestone.md": 31134,
+  "complete-milestone.md": 31027,
   "debug.md": 13505,
   "diagnose-issues.md": 12820,
   "discovery-phase.md": 8651,
@@ -44,7 +44,7 @@
   "map-codebase.md": 20789,
   "milestone-summary.md": 11774,
   "mvp-phase.md": 13582,
-  "new-milestone.md": 32422,
+  "new-milestone.md": 32844,
   "new-project.md": 66138,
   "new-workspace.md": 11254,
   "next.md": 20094,