docs(platform): Compass Model Roles + stable-name provider routing design (RIG-2845)

[rigel-mintaka]

The policy layer above the frozen RIG-1715 gateway credential substrate:
which model a Compass agent role uses, and how a stable model name maps to a
preferred backend order across the providers a user holds.

Core decisions:

• Role taxonomy: Compass-native persona-keyed roles ABOVE OMP's 10 model-tier
  roles (untouched). Policy = CompassRole -> {stable name, thinking?,
  tierOverrides?}; main model rides the existing COMPASS_MODEL seam,
  tierOverrides ride the config-passthrough CP-1 modelRoles key (no schema
  fork).
• Stable names (claude-opus-4-8) over direct backend routing: each maps to an
  ordered candidate chain, two-stage fold (RIG-2845 orders providers ->
  RIG-1715 pool picks credential verbatim); unknown names pass through as the
  escape hatch.
• Config surface: Compass-owned Postgres policy store + compass.v1 RPCs.

Red-teamed by a design-critic pass and folded before freeze. Two structural
corrections the critique surfaced, both verified at source:

• The resolver seam is widened (async + caller identity), not zero-fork: the
  injected resolveModel(modelId) is sync + tenant-blind and cannot carry
  per-caller candidate selection. Framed as OQ-1 (sibling of the RIG-1715 T3
  authorize boot-option change).
• A stable name exported as COMPASS_MODEL must ALSO resolve inside the
  container (cli.ts:991-1004 refuse-to-boot); added a container-registry
  listing deliverable (OQ-2).

Five load-bearing Open Questions batched for Matt (seam shape, container
registry, failover trigger, write authority, policy scope).

Ledger-impact: none (platform surface ungoverned).
Spec-impact: none. Refs RIG-2845

Co-authored-by: Matt Wilkinson matt@rigel.build

[linear-code]

RIG-2845

[github-actions]

Compass engineering docs preview: https://compass-obs-rig-2845-model-r.compass-eng-docs.pages.dev

Deployed from compass-obs/rig-2845-model-roles-design at 584560a.

Changed pages:

• docs/designs/platform/compass-model-roles/design.md