fix(cli): address review findings on prompt-snapshot [FR-004]

- correct stale header comment (--check is implemented, not "a later task")
- narrow @Format against the render escaper registry (fmtAttr in ESCAPERS) so
  the RenderFormat cast is checked, not an unsafe assertion; keep the conditional
  spread (exactOptionalPropertyTypes forbids `format: undefined`)
- add a --check render-error coverage test (unresolvable @textRef -> exit 1)

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: dmealing

server/typescript/packages/cli/src/commands/prompt-snapshot.ts

@@ -3,9 +3,10 @@
 // For each template.* node with a committed fixture payload, render its @textRef
 // text against that payload (same engine, provider, and @format escaping prod
 // uses) and snapshot the byte-exact output under .metaobjects/snapshots/<name>/.
-// Write mode (default) overwrites output.snap; --check (a later task) diffs and
-// fails on drift. Closes the gap the template's own git history misses: a shared
-// partial or payload-shape change that silently alters the rendered prompt.
+// Write mode (default) overwrites output.snap; --check compares against the
+// committed golden and exits 1 on drift (never writes). Closes the gap the
+// template's own git history misses: a shared partial or payload-shape change
+// that silently alters the rendered prompt.
 
 import { join } from "node:path";
 import { existsSync, readFileSync, mkdirSync, writeFileSync } from "node:fs";
@@ -15,7 +16,7 @@ import { FileProvider } from "../lib/file-provider.js";
 import { snapshotPaths, unifiedDiff } from "../lib/snapshot.js";
 import { loadMemory } from "@metaobjectsdev/sdk";
 import { TYPE_TEMPLATE, TEMPLATE_ATTR_TEXT_REF, TEMPLATE_ATTR_FORMAT } from "@metaobjectsdev/metadata";
-import { render, type RenderFormat } from "@metaobjectsdev/render";
+import { render, ESCAPERS, type RenderFormat } from "@metaobjectsdev/render";
 
 const DEFAULT_PROMPTS_DIR = "prompts";
 
@@ -77,8 +78,13 @@ export async function promptSnapshotCommand(args: string[], cwd: string): Promis
       continue;
     }
 
+    // @format is loader-validated against the template format vocabulary; narrow
+    // against the render engine's own escaper registry so the RenderFormat cast is
+    // a checked narrowing (never a TypeError on an unknown format), and omit the
+    // key entirely when absent (exactOptionalPropertyTypes forbids `format: undefined`).
     const fmtAttr = tmpl.ownAttr(TEMPLATE_ATTR_FORMAT);
-    const format = typeof fmtAttr === "string" ? (fmtAttr as RenderFormat) : undefined;
+    const format =
+      typeof fmtAttr === "string" && fmtAttr in ESCAPERS ? (fmtAttr as RenderFormat) : undefined;
 
     let rendered: string;
     try {

server/typescript/packages/cli/test/integration/prompt-snapshot.test.ts

@@ -159,4 +159,17 @@ describe("meta prompt-snapshot --check", () => {
       rmSync(tmp, { recursive: true, force: true });
     }
   });
+
+  test("exit 1 when a template's @textRef does not resolve (render error path)", async () => {
+    const tmp = scaffold();
+    writePayload(tmp, "farewell", { name: "Ada" });
+    await run(["prompt-snapshot", "--cwd", tmp]); // write the golden first
+    rmSync(join(tmp, "prompts", "prompt", "farewell.mustache"), { force: true }); // now unresolvable
+    try {
+      expect(await run(["prompt-snapshot", "--check", "--cwd", tmp])).toBe(1);
+      expect([...out, ...err].join("\n")).toContain("farewell");
+    } finally {
+      rmSync(tmp, { recursive: true, force: true });
+    }
+  });
 });