fix(#2118): honor --dry-run in milestone complete with zero-mutation preview (#2155)

* fix(2118): honor --dry-run in milestone complete with zero-mutation preview

milestone complete treated --dry-run as a no-op: the flag was neither
parsed nor rejected, so a caller who expected a preview instead
triggered the full destructive mutation (archive phases → move audit
artifacts → rewrite STATE.md) with no way to back out.

Fix (option 2 from the issue): add dryRun to MilestoneCompleteOptions,
parse --dry-run in the dispatcher, and return a JSON preview plan
(would_archive, would_update) after the read-only stats gathering but
before any mutations. Also gated platformEnsureDir on !dryRun so the
archive directory is not created during preview.

3 regression tests: no-mutation happy path, --no-archive-phases combo,
and --force bypass combo.

* docs(changeset): backfill PR number (#2155)

* chore(#2118): regenerate pi/qwen golden fixtures after next merge

The milestone --dry-run fix changes gsd-core/bin/gsd-tools.cjs; qwen.json
(missed at authoring) and pi.json (added on next, never carried the fix)
were the only two runtime fixtures still holding the pre-fix hash. All
other runtimes already reflect the change. Regenerated via the sanctioned
gen-golden-install-parity script.

* fix(#2118): surface accomplishments in dry-run preview; fix --dry-run --raw

Orthogonal review findings on the --dry-run preview:
- The preview omitted the already-computed accomplishments (the primary
  MILESTONES.md content a real run writes); surface it as a top-level field,
  mirroring the real-run result.
- `--dry-run --raw` discarded the structured payload and printed the literal
  string "dry-run"; drop the raw-value arg so --raw emits the full preview
  JSON, matching the real-run output() call.
Adds tests: --dry-run --raw is parseable JSON, preview includes accomplishments,
and --dry-run --force is proven zero-mutation.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

.changeset/2118-milestone-complete-dry-run.md

@@ -0,0 +1,5 @@
+---
+type: Fixed
+pr: 2155
+---
+**`milestone complete --dry-run` now prints a preview plan instead of silently mutating** — `gsd-tools milestone complete --dry-run` was neither parsed nor rejected, so a caller expecting a preview triggered the full destructive mutation (archive phases, move audit artifacts, rewrite STATE.md) with no way to back out. The `--dry-run` flag is now honored: it returns a JSON plan listing `would_archive` (roadmap, requirements, audit, phase dirs) and `would_update` (MILESTONES.md, STATE.md) targets with zero filesystem mutations. (#2118)