feat(#1517): support custom reviewer instances for /gsd:review (#1766)

* feat(#1517): support custom reviewer instances for /gsd:review

Add a bounded review.reviewer_instances config surface so one model-capable
adapter (e.g. opencode) can run as several independent reviewer identities in a
single /gsd:review pass. Instances participate only via review.default_reviewers,
expand before built-in slugs, are available iff their cli is detected, and a
non-matching entry is a hard error (typo must be loud). >=2 same-cli instances
emit a shared-adapter caveat in REVIEWS.md. Default path with no instances is
byte-for-byte unchanged.

Single-source instance->cli resolution lives in resolveReviewerSelection /
normalizeReviewerInstances (parity-locked in
tests/review-reviewer-instances.test.cjs). cli validated against
KNOWN_REVIEWER_SLUGS only (never arbitrary shell); model/agent opaque, never
shell-interpolated.

Closes #1517

* chore(#1517): backfill changeset pr:1766

---------

Co-authored-by: review-bot <review-bot@gsd>

commands/gsd/review.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>