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>

docs/INVENTORY-MANIFEST.json

@@ -248,6 +248,7 @@
       "research-documentation-lookup.md",
       "research-philosophy.md",
       "research-verification-protocol.md",
+      "reviewer-instances.md",
       "revision-loop.md",
       "scout-codebase.md",
       "security-asvs-levels.md",