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

gsd-core/workflows/help/modes/full.md

@@ -673,7 +673,7 @@ These six skills exist primarily for the model to perform two-stage hierarchical
 ├── milestones/
 │   ├── v1.0-ROADMAP.md       # Archived roadmap snapshot
 │   ├── v1.0-REQUIREMENTS.md  # Archived requirements
-│   └── v1.0-phases/          # Archived phase dirs (via /gsd:cleanup or --archive-phases)
+│   └── v1.0-phases/          # Archived phase dirs (via /gsd:cleanup or milestone complete, which archives by default)
 │       ├── 01-foundation/
 │       └── 02-core-features/
 ├── codebase/             # Codebase map (brownfield projects)