feat(#903): generate Loop Host Contract from workflow markers (ADR-857 phase 3a-impl-2) (#906)

Replace the inline LOOP_HOST_CONTRACT constant in the Capability Registry
generator with a generated-from-workflows contract (ADR-894 §3). The contract
is now derived from inert `<!-- gsd:loop-host ... -->` marker blocks in the five
step workflows, emitted as the committed gsd-core/bin/lib/loop-host-contract.cjs,
and required by gen-capability-registry.cjs — one source of truth, no drift.

Drift guards in gen-loop-host-contract.cjs: per-step point ownership (each step
must declare exactly its canonical loop points), multiple-block + duplicate-key
hard errors, and a word-boundary agent-role cross-check. Contract content is
byte-identical to the former constant; registry-only, nothing wired into the
live loop.

Closes #903

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

docs/ARCHITECTURE.md

@@ -369,6 +369,7 @@ Node.js CLI utility (`gsd-tools.cjs`) with domain modules split across `gsd-core
 | `schema-detect.cjs`    | Schema-drift detection for ORM patterns (Prisma, Drizzle, etc.)                                     |
 | `profile-pipeline.cjs` | User behavioral profiling data pipeline, session file scanning                                      |
 | `profile-output.cjs`       | Profile rendering, USER-PROFILE.md and dev-preferences.md generation                                |
+| `loop-host-contract.cjs`   | Generated Loop Host Contract — 12 loop points, per-step agent roles, and core artifacts; emitted by `scripts/gen-loop-host-contract.cjs` from workflow markers (ADR-894 §3); consumed by `gen-capability-registry.cjs` |
 | `capability-registry.cjs`  | Generated central Capability Registry — role-partitioned index of all co-located capability declarations; emitted by `scripts/gen-capability-registry.cjs` (ADR-894 §5) |