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>

package.json

@@ -77,10 +77,11 @@
     "check:alias-drift": "node scripts/check-alias-drift.cjs",
     "check:identity-drift": "node scripts/lint-package-identity-drift.cjs",
     "check:integrity": "node scripts/check-npm-integrity.cjs",
-    "build": "npm run generate:identity && npm run build:lib && npm run gen:capability-registry && npm run build:hooks",
+    "build": "npm run generate:identity && npm run build:lib && npm run gen:loop-host-contract && npm run gen:capability-registry && npm run build:hooks",
     "build:hooks": "node scripts/build-hooks.js",
     "build:lib": "tsc -p tsconfig.build.json",
     "generate:identity": "node scripts/generate-package-identity.cjs",
+    "gen:loop-host-contract": "node scripts/gen-loop-host-contract.cjs --write",
     "gen:capability-registry": "node scripts/gen-capability-registry.cjs --write",
     "prepack": "npm run build:lib",
     "prepare": "npm run build:lib",