The discovery spine now reaches the build. Registered DOMAIN-MODEL.md, the architecture
ADR, and TEST-STRATEGY.md into discuss-phase's canonical_refs (gsd's "downstream agents
MUST read these" contract), and hardened plan-phase so the planner + plan-checker actually
open canonical references (with a direct-read fallback for non-discuss CONTEXT paths).

- gsd-core/workflows/discuss-phase.md: register the 3 project-level discovery artifacts as canonical refs
- gsd-core/workflows/plan-phase.md: planner/checker must open canonical refs + read the artifacts directly if present
- Proven via adversarial dry-run: same phase planned with vs without artifacts yields materially
  different plans (Domain Model + hexagonal vs CRUD controller; integer cents vs float; Testcontainers
  integration tests with live @-refs vs mocked; e2e smoke vs none).
- Also fixes a latent gsd-core gap: canonical_refs were stated MUST-read but never explicitly opened.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>