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/execute-phase.md

@@ -1,3 +1,10 @@
+<!-- gsd:loop-host
+step: execute
+points: execute:pre, execute:wave:pre, execute:wave:post, execute:post
+agent-roles: executor, verifier
+produces: SUMMARY.md
+consumes: PLAN.md
+-->
 <purpose>
 Execute all plans in a phase using wave-based parallel execution. Orchestrator stays lean — delegates plan execution to subagents.
 </purpose>