scripts(typing[mcp_swap]): narrow claude project node via isinstance

CI's ``uv run mypy .`` flagged
``Returning Any from function declared to return "dict[str, Any] | None"``
at the return of ``_claude_project_node``: ``projects.get(key)`` produces
``Any`` and the existing inner shape guard didn't propagate that
narrowing to the return site.

Replace the post-hoc check with an isinstance-narrowed assignment:
``raw_node`` stays ``Any`` from ``projects.get`` but the
``isinstance(raw_node, dict)`` branch narrows it onto a typed
``node: dict[str, t.Any] | None``. Same runtime behaviour (the
non-None / non-dict path still raises ``RuntimeError``) without
``t.cast``.

This supersedes a transient ``t.cast`` fix on this branch.

Author: tony

scripts/mcp_swap.py

@@ -275,12 +275,15 @@ def _claude_project_node(
     projects = (
         config.setdefault("projects", {}) if create else config.get("projects", {})
     )
-    node = projects.get(key)
-    if node is not None and not isinstance(node, dict):
+    raw_node = projects.get(key)
+    node: dict[str, t.Any] | None = None
+    if isinstance(raw_node, dict):
+        node = raw_node
+    elif raw_node is not None:
         msg = (
             "Claude config layout appears to have changed; expected "
             f"'projects[{key!r}]' to be a mapping but got "
-            f"{type(node).__name__}"
+            f"{type(raw_node).__name__}"
         )
         raise RuntimeError(msg)
     if node is None and create: