chore: add check for rpc url in pre-commit (#53)

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<!-- CURSOR_SUMMARY -->
---

> [!NOTE]
> Adds a local pre-commit hook that blocks changes to the `url` line in `Scarb.toml` by inspecting the staged diff.
> 
> - **Tooling / Pre-commit**:
>   - Add local hook `scarb-toml-url-lock` in `.pre-commit-config.yaml` targeting `Scarb.toml` at commit stage.
>   - Introduce `scripts/check_scarb_url_lock.py` to inspect the staged diff of `Scarb.toml` and fail if `+url` or `-url` changes are detected.
> 
> <sup>Written by [Cursor Bugbot](https://cursor.com/dashboard?tab=bugbot) for commit c44bef8. This will update automatically on new commits. Configure [here](https://cursor.com/dashboard?tab=bugbot).</sup>
<!-- /CURSOR_SUMMARY -->

Author: arad-starkware

.pre-commit-config.yaml

@@ -0,0 +1,10 @@
+repos:
+  - repo: local
+    hooks:
+      - id: scarb-toml-url-lock
+        name: forbid Scarb.toml rpc url change
+        entry: scripts/check_scarb_url_lock.py
+        language: system
+        pass_filenames: false
+        files: ^Scarb\.toml$
+        stages: [commit]

scripts/check_scarb_url_lock.py

@@ -0,0 +1,48 @@
+#! /usr/bin/env python3
+"""Fail if the Scarb.toml rpc url line is staged for commit."""
+
+from __future__ import annotations
+
+import subprocess
+import sys
+
+
+def staged_diff(path: str) -> str:
+    """Return the staged diff for `path` or an empty string."""
+
+    result = subprocess.run(
+        ["git", "diff", "--cached", "--unified=0", "--", path],
+        check=False,
+        capture_output=True,
+        text=True,
+    )
+
+    if result.returncode != 0:
+        # `git diff` returns 0 on success; any other code indicates an error.
+        print("pre-commit: failed to inspect staged diff for Scarb.toml", file=sys.stderr)
+        print(result.stderr.strip(), file=sys.stderr)
+        sys.exit(result.returncode)
+
+    return result.stdout
+
+
+def main() -> int:
+    diff = staged_diff("Scarb.toml")
+    if not diff:
+        return 0
+
+    for line in diff.splitlines():
+        if not line or line[0] not in {"+", "-"}:
+            continue
+        if line.startswith(("+url", "-url")):
+            print(
+                "pre-commit: Scarb.toml rpc url must not change; revert or drop it before committing.",
+                file=sys.stderr,
+            )
+            return 1
+
+    return 0
+
+
+if __name__ == "__main__":
+    sys.exit(main())