chore: regenerate stale gsd-review SKILL.md (pre-existing next drift)

gen:plugin-skills regenerates skills/gsd-review/SKILL.md from source; next's
committed copy was stale (missing the review.default_reviewers 'No flags' block
present in source). Surfaced by the full build during this forward-port. Not
gated by CI (test.yml runs only build:lib), so it had drifted silently.
Deterministic regen; the only generated file out of sync.

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

skills/gsd-review/SKILL.md

@@ -35,6 +35,12 @@ Phase number: extracted from $ARGUMENTS (required)
 - `--cursor` — Include Cursor agent review
 - `--agy` / `--antigravity` — Include Antigravity CLI review
 - `--all` — Include all available CLIs
+
+**No flags** — if `review.default_reviewers` is set, review with only those configured
+reviewers that are detected; otherwise review with all available CLIs. Configured
+`review.reviewer_instances` names may appear in `review.default_reviewers`; each runs as an
+independent reviewer identity backed by its configured adapter+model (see
+`docs/CONFIGURATION.md`). Instance names are not valid as flags.
 </context>
 
 <process>