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.md

@@ -312,6 +312,7 @@ Full roster at `gsd-core/references/*.md`. References are shared knowledge docum
 | `loop-hook-dispatch.md` | Generic dispatch contract for consuming `gsd_run loop render-hooks <point> --raw` output in any host-loop workflow — envelope shape, per-kind dispatch rules (contribution/step/gate), and liveness banner. |
 | `scout-codebase.md` | Phase-type→codebase-map selection table for discuss-phase scout step (extracted via the discuss-phase/modes progressive-disclosure split, #717). |
 | `revision-loop.md` | Plan revision iteration patterns. |
+| `reviewer-instances.md` | Custom reviewer instances for `/gsd-review` (#1517) — same-adapter multi-model review: config shape, resolution rules, invocation, and the REVIEWS.md contract. Lazily loaded by `review.md` when `review.reviewer_instances` is configured. |
 | `universal-anti-patterns.md` | Universal anti-patterns to detect and avoid. |
 | `worktree-branch-check.md` | Canonical spawn-time worktree HEAD/base guard (worktree_branch_check): verify-only and fail-closed — per-agent-branch assertion, protected-ref refusal (#2924), and an exact-base assertion that halts with `exit 42` on mismatch so the orchestrator (worktree lifecycle owner) performs recovery (#48). Embedded into worktree sub-agent prompts at dispatch. |
 | `worktree-path-safety.md` | Worktree guard suite: HEAD assertion, cwd-drift sentinel (step 0a, #3097), and absolute-path guard (step 0b, #3099) — loaded into executor spawn prompts via `<execution_context>`. |