[RNE Rewrite] chore: set up clangd for C++ sources

[msluszniak]

Description

Sets up a committed clangd config for the core package's C/C++ sources so the rewrite gets code intelligence and a shared, strict warning set, and enforces that set at commit time. First step of #1271; clang-tidy CI is a follow-up (#1286).

• compile_flags.txt — include roots / -std=c++20 / defines, with third-party headers (ExecuTorch/torch/JSI) as -isystem so their warnings don't pollute our diagnostics. Relative paths anchored to the package dir, so it is portable with no per-machine generation.
• .clangd — strict warning set layered on top (incl. -Wconversion/-Wsign-conversion), with clangd's include cleaner disabled (ExecuTorch umbrella headers misreport includes).
• scripts/check-cpp-warnings.sh + lefthook.yml — a pre-commit command that compiles staged cpp/ sources with the same warning set and aborts the commit on any warning, keeping the editor and the commit gate in sync. It skips gracefully (never blocks) when no compiler or the provisioned headers are available; bypass with git commit --no-verify.
• .gitignore — track the shared config while keeping generated compile_commands.json local.
• CONTRIBUTING.md — prerequisites (provisioned third-party/include + yarn install), editor setup, and the pre-commit behavior.

Introduces a breaking change?

• Yes
• No

Type of change

• Bug fix (change which fixes an issue)
• New feature (change which adds functionality)
• Documentation update (improves or adds clarity to existing documentation)
• Other (chores, tests, code style improvements etc.)

Tested on

• iOS
• Android

Testing instructions

1. Provision packages/react-native-executorch/third-party/include and run yarn install.
2. Open a file under packages/react-native-executorch/cpp in an editor with the clangd extension (MS C/C++ IntelliSense disabled); confirm the <executorch/...>/<jsi/jsi.h> includes resolve and the current sources are clean.
3. Confirm the strict flags fire by introducing a deliberate violation, e.g. in cpp/core/dtype.cpp:

   int rne_probe(float f) {
       int casted = (int)f;   // expect -Wold-style-cast + -Wconversion
       int unused = 7;        // expect -Wunused-variable
       return casted;
   }

   clangd should flag these on our code (third-party headers stay quiet thanks to -isystem).
4. git add that change and git commit — the pre-commit hook aborts with the warning printed. Verified end-to-end via lefthook: warning → cpp-warnings ❌ (exit 1); clean → ✔️ (exit 0). Revert afterwards.

Related issues

#1271

Checklist

• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have updated the documentation accordingly
• My changes generate no new warnings