fix: unify $status to const-only, drop enum support (#123)

- Validator: hasStatusConst/getConstStatuses replace enum checks
- enum in $status is now rejected with clear error message
- All docs/examples/tests migrated from enum to const/oneOf
- bootstrap hello.yaml updated

Fixes #123

packages/util-agent/__tests__/build-output-format-instruction.test.ts

@@ -143,7 +143,7 @@ describe("buildOutputFormatInstruction", () => {
         {
           type: "object",
           properties: {
-            $status: { type: "string", enum: ["approved"] },
+            $status: { const: "approved" },
             branch: { type: "string" },
           },
           required: ["$status"],
@@ -151,7 +151,7 @@ describe("buildOutputFormatInstruction", () => {
         {
           type: "object",
           properties: {
-            $status: { type: "string", enum: ["rejected"] },
+            $status: { const: "rejected" },
             comments: { type: "string" },
           },
           required: ["$status"],