refactor(mcp): single docmostSchema + shared encode-error helper + catch test (#152 review)

Review of #154 (Request changes) — all clean follow-ups, no defect in the fix:

1. Single source of the ProseMirror schema: export `docmostSchema` from
   docmost-schema.ts (next to docmostExtensions); diff.ts and collaboration.ts
   import it instead of each calling getSchema(docmostExtensions) — the schema
   can no longer drift between call sites. Removed both local builds + the now
   unused getSchema imports.
2. Doc fix: assertYjsEncodable's docstring and the client.ts comment no longer
   claim "the same encoder as apply" — apply uses updateYFragment, the dry-run
   uses toYdoc; both reject the same unstorable attrs but are NOT byte-identical.
   Reworded to "independent encodability gate".
3+4+5. Extracted `unstorableYjsError(safe, label, e)` — buildYDoc and
   applyDocToFragment now share one message template (label kept for diagnostics:
   toYdoc vs updateYFragment), so the wording can't drift between dry-run/apply.
6. Test for applyDocToFragment's catch branch: an unknown node type makes the
   schema-validated PMNode.fromJSON throw, and the function must re-throw it
   wrapped with the (updateYFragment) diagnostic.

build/ rebuilt for the three changed lib modules; 293 package tests green.
(Left build/client.js untouched: rebuilding it would pull in a pre-existing,
unrelated src/build drift — a listSidebarPages slugId fix never rebuilt on
develop — and my client.ts change there is comment-only.)

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

packages/mcp/test/unit/comment-cursor-stability.test.mjs

@@ -73,3 +73,21 @@ test("anchoring a comment mark keeps the cursor in the marked text (issue #152)"
   const abs = Y.createAbsolutePositionFromRelativePosition(relPos, ydoc);
   assert.notEqual(abs, null, "comment anchoring must not destroy the cursor anchor");
 });
+
+// The diagnostic catch branch of applyDocToFragment (#154 review): a doc that
+// cannot be hydrated/encoded must be re-thrown wrapped with the stage label, not
+// leak the raw ProseMirror/Yjs error. An unknown node type makes
+// PMNode.fromJSON (against the docmost schema) throw — a reliable trigger
+// (sanitizeForYjs only strips `undefined`, so an undefined attr would be removed
+// before it could fail).
+test("applyDocToFragment wraps an encode/build failure with the (updateYFragment) diagnostic", () => {
+  const ydoc = new Y.Doc();
+  const bad = {
+    type: "doc",
+    content: [{ type: "totally_unknown_node_xyz_12345" }],
+  };
+  assert.throws(
+    () => applyDocToFragment(ydoc, bad),
+    /Failed to encode document to Yjs \(updateYFragment\)/,
+  );
+});