Seed bundled demos under top-level demos/ instead of code/

Move canonical sample scripts to a sibling folder of code/ and lib/ so
user projects stay separate from shipped examples. Backend seeding,
writable paths, and docs follow the new layout.

Co-authored-by: Cursor <cursoragent@cursor.com>

src/editor_app/services/filesystem.py

@@ -7,7 +7,7 @@ from editor_app import config
 from editor_app.models import FileInfo
 
 LIB_DIR_NAME = "lib"
-WRITABLE_ROOTS = {"code"}
+WRITABLE_ROOTS = {"code", "demos"}
 
 
 def _workspace_root(workspace_root: Path | None = None) -> Path:
@@ -33,6 +33,9 @@ def normalize_relative_path(relative_path: str) -> str:
     if len(parts) >= 2 and parts[0] == "code":
         while len(parts) >= 2 and parts[0] == parts[1] == "code":
             parts.pop(1)
+    if len(parts) >= 2 and parts[0] == "demos":
+        while len(parts) >= 2 and parts[0] == parts[1] == "demos":
+            parts.pop(1)
     return "/".join(parts)
 
 
@@ -84,7 +87,7 @@ def _ensure_writable_path(target_path: Path, workspace_root: Path | None = None)
     if not _is_writable_path(target_path, workspace_root):
         raise HTTPException(
             status_code=403,
-            detail="Only code/ is writable (lib is read-only)",
+            detail="Only code/ and demos/ are writable (lib is read-only)",
         )