refactor(cli): add stderr diagnostics logger + oxlint guard (#2551)

packages/cli/src/ui/diagnostics.ts

@@ -0,0 +1,23 @@
+/**
+ * CLI diagnostics logger.
+ *
+ * Diagnostics (notices, progress, warnings, errors) go to **stderr**. stdout is
+ * reserved for a command's machine-readable payload — most importantly `--json`
+ * output — and its primary human result. A diagnostic written to stdout corrupts
+ * that payload for any consumer that parses it (see #2520 / #2522).
+ *
+ * This mirrors the producer's `ProducerLogger` contract for the CLI, but without
+ * `[LEVEL]` prefixes so user-facing notices keep their own formatting. Route all
+ * diagnostic output through `diag` (or `console.error` / `console.warn` directly)
+ * — never `console.log` / `console.info` for diagnostics.
+ */
+export const diag = {
+  /** A user-facing notice or progress line (e.g. "Installing … skills…"). */
+  notice(...args: unknown[]): void {
+    console.error(...args);
+  },
+  /** A warning the user should see but that isn't fatal. */
+  warn(...args: unknown[]): void {
+    console.warn(...args);
+  },
+};