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>

gsd-core/bin/shared/config-schema.manifest.json

@@ -172,6 +172,11 @@
       "source": "^review\\.max_prompt_tokens_per_reviewer\\.[a-zA-Z0-9_-]+$",
       "description": "review.max_prompt_tokens_per_reviewer.<reviewer-slug>"
     },
+    {
+      "topLevel": "review",
+      "source": "^review\\.reviewer_instances\\.[a-zA-Z0-9_-]+\\.(cli|model|agent)$",
+      "description": "review.reviewer_instances.<instance-name>.<cli|model|agent> (#1517)"
+    },
     {
       "topLevel": "model_policy",
       "source": "^model_policy\\.runtime_tiers\\.[a-zA-Z0-9_-]+\\.(opus|sonnet|haiku)$",