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

sdk/src/query/config-schema.ts

@@ -48,6 +48,7 @@ export const VALID_CONFIG_KEYS: ReadonlySet<string> = new Set([
   'ship.pr_body_sections',
   'git.branching_strategy', 'git.base_branch', 'git.phase_branch_template', 'git.milestone_branch_template', 'git.quick_branch_template',
   'planning.commit_docs', 'planning.search_gitignored', 'planning.sub_repos',
+  'review.default_reviewers',
   'review.ollama_host', 'review.lm_studio_host', 'review.llama_cpp_host',
   'workflow.cross_ai_execution', 'workflow.cross_ai_command', 'workflow.cross_ai_timeout',
   'workflow.subagent_timeout',