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

src/phases-command-router.cts

@@ -3,8 +3,8 @@
  * Keeps gsd-tools.cjs thin while preserving current CJS semantics.
  *
  * Unsupported in this router (treated as unknown):
- * - archive: `phases archive` is excluded from the subcommands list so it
- *   falls through to the unknown-subcommand error path.
+ * - archive: `phases archive` is excluded (#2684) — it is an internal
+ *   subcommand that milestone.complete forwards to, not a public surface.
  *
  * ADR-457 build-at-publish: the hand-written bin/lib/phases-command-router.cjs
  * collapsed to a TypeScript source of truth. Behaviour is preserved byte-for-behaviour
@@ -46,7 +46,8 @@ interface RoutePhasesCommandOptions {
 function routePhasesCommand({ phase, milestone, args, cwd, raw, error }: RoutePhasesCommandOptions): void {
   routeCjsCommandFamily({
     args,
-    // Exclude 'archive' so it hits the unknownMessage path.
+    // #2684: `phases archive` is deliberately excluded — it is an internal
+    // subcommand that milestone.complete forwards to, not a public surface.
     subcommands: PHASES_SUBCOMMANDS.filter((s) => s !== 'archive'),
     error,
     unknownMessage: (_subcommand: string, available: string[]) => `Unknown phases subcommand. Available: ${available.join(', ')}`,