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>

gsd-core/workflows/plan-phase.md

@@ -1,3 +1,10 @@
+<!-- gsd:loop-host
+step: plan
+points: plan:pre, plan:post
+agent-roles: researcher, planner, checker
+produces: PLAN.md
+consumes: CONTEXT.md
+-->
 <purpose>
 Create executable phase prompts (PLAN.md files) for a roadmap phase with integrated research and verification. Default flow: Research (if needed) -> Plan -> Verify -> Done. Orchestrates gsd-phase-researcher, gsd-planner, and gsd-plan-checker agents with a revision loop (max 3 iterations).
 </purpose>