feat(protocol): create @uncaged/workflow-protocol package

Extract all cross-package type definitions and constructor functions
into a dedicated protocol layer. This is the foundation for the
seven-package split (RFC #143).

Contains:
- Result<T,E>, ok(), err()
- START, END constants
- CasStore, WorkflowFn, RoleOutput, WorkflowCompletion
- WorkflowDescriptor, WorkflowRoleDescriptor
- ProviderConfig, WorkflowConfig, ResolvedModel (fixes config↔registry cycle)
- RoleDefinition, Moderator, WorkflowDefinition
- AgentFn, ExtractFn, and all thread context types

Ref: #143, closes #144

packages/workflow-protocol/package.json

@@ -0,0 +1,18 @@
+{
+  "name": "@uncaged/workflow-protocol",
+  "version": "0.1.0",
+  "type": "module",
+  "exports": {
+    ".": {
+      "types": "./dist/index.d.ts",
+      "import": "./src/index.ts"
+    }
+  },
+  "peerDependencies": {
+    "zod": "^4.0.0"
+  },
+  "devDependencies": {
+    "zod": "^4.0.0",
+    "typescript": "^5.8.3"
+  }
+}