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

get-shit-done/workflows/review.md

@@ -49,7 +49,19 @@ Parse flags from `$ARGUMENTS`:
 - `--lm-studio` → include LM Studio (local server, OpenAI-compatible)
 - `--llama-cpp` → include llama.cpp (local server, OpenAI-compatible)
 - `--all` → include all available (CLIs + running local servers)
-- No flags → include all available
+- No flags → if `review.default_reviewers` is set, include only configured reviewers that are detected; otherwise include all available
+
+Reviewer-selection precedence:
+1. Individual reviewer flags (`--gemini`, `--codex`, etc.)
+2. `--all`
+3. `review.default_reviewers`
+4. No key + no flags → all detected reviewers
+
+`review.default_reviewers` behavior:
+- Value must be a non-empty array of slug strings (configured via `gsd config-set review.default_reviewers '["gemini","codex"]'`)
+- Unknown slugs warn and are ignored
+- Known-but-undetected slugs emit an info note and are ignored
+- If all configured reviewers are unavailable, fail with an actionable message
 
 If no CLIs are available:
 ```