feat: add review.default_reviewers config for /gsd-review defaults (#3464)

* feat(review): add review.default_reviewers selection policy

* docs(review): explain default reviewer config and precedence

* chore(changeset): add feature entry for review.default_reviewers

* chore(changeset): set pr field for #3464

* test(review): cover unavailable default-reviewer failure path

* fix(review): sync sdk and inventory parity for default reviewers

* fix(review): use canonical /gsd:review namespace in source

docs/INVENTORY-MANIFEST.json

@@ -1,5 +1,5 @@
 {
-  "generated": "2026-05-12",
+  "generated": "2026-05-13",
   "families": {
     "agents": [
       "gsd-advisor-researcher",
@@ -295,6 +295,7 @@
       "planning-workspace.cjs",
       "profile-output.cjs",
       "profile-pipeline.cjs",
+      "review-reviewer-selection.cjs",
       "roadmap-command-router.cjs",
       "roadmap.cjs",
       "runtime-homes.cjs",