ci: pin Node 22 via root .nvmrc for the workspace-source build

Re-lands the Node pin that was lost when #5534 merged without it. The
Railway slack-example build now compiles the @copilotkit/* libs from source
(tsdown/rolldown), which needs Node >=20.12 (`node:util`'s `styleText`), but
the builder defaults to Node 18 from the root `engines.node: ">=18"` —
failing with "does not provide an export named 'styleText'". Pin the
build/dev Node to 22 (read by the Railway builder and nvm/fnm). CI is
unaffected (no workflow reads .nvmrc) and the published `engines` runtime
contract is left as-is.

.nvmrc

@@ -0,0 +1 @@
+22