chore: add CLAUDE.md, clang-tidy, and agent workflow tooling (#419)

## Summary
- Add `.claude/CLAUDE.md` with project guide, C++ coding style,
correction patterns from past AI interactions, and pre-commit/pre-PR
workflows
- Add `.claude/commands/` with `/build`, `/test`, `/format` slash
commands for Claude Code
- Add `.clang-tidy` configuration
- Add `scripts/run_clang_tidy.py` for parallel clang-tidy with progress
reporting (reads CDB, filters to `src/` and `tests/` only)
- Add pixi tasks: `clang-tidy`, `lint-cpp`, `lint-python`, `lint`
- Add `clang-tools` dependency for clang-tidy
- Update `.gitignore` to track `.claude/CLAUDE.md` and
`.claude/commands/`

## Test plan
- [x] `pixi run format` passes
- [x] `pixi run lint-python` passes (ruff check)
- [x] `scripts/run_clang_tidy.py` correctly filters to project files
only

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

scripts/run_clang_tidy.py

@@ -0,0 +1,66 @@
+#!/usr/bin/env python3
+"""Run clang-tidy in parallel on all files in compile_commands.json."""
+
+import json
+import subprocess
+import sys
+import threading
+from concurrent.futures import ThreadPoolExecutor, as_completed
+from pathlib import Path
+
+
+def main():
+    build_dir = sys.argv[1] if len(sys.argv) > 1 else "build/RelWithDebInfo"
+    cdb_path = Path(build_dir) / "compile_commands.json"
+
+    if not cdb_path.exists():
+        print(f"Error: {cdb_path} not found. Run cmake-config first.", file=sys.stderr)
+        sys.exit(1)
+
+    project_root = Path(__file__).resolve().parent.parent
+    src_dirs = (project_root / "src", project_root / "tests")
+
+    cdb = json.loads(cdb_path.read_text())
+    files = [
+        entry["file"]
+        for entry in cdb
+        if any(Path(entry["file"]).resolve().is_relative_to(d) for d in src_dirs)
+    ]
+    total = len(files)
+
+    lock = threading.Lock()
+    done = 0
+    failed = []
+
+    def run(file: str) -> tuple[str, int, str]:
+        result = subprocess.run(
+            ["clang-tidy", "-p", build_dir, "--quiet", file],
+            capture_output=True,
+            text=True,
+        )
+        return file, result.returncode, result.stdout + result.stderr
+
+    with ThreadPoolExecutor() as pool:
+        futures = {pool.submit(run, f): f for f in files}
+        for future in as_completed(futures):
+            file, code, output = future.result()
+            with lock:
+                done += 1
+                name = Path(file).name
+                if code != 0:
+                    failed.append(file)
+                    print(f"[{done}/{total}] FAIL {name}")
+                    if output.strip():
+                        print(output, end="")
+                else:
+                    print(f"[{done}/{total}] OK   {name}")
+
+    if failed:
+        print(f"\nclang-tidy failed on {len(failed)}/{total} files.", file=sys.stderr)
+        sys.exit(1)
+
+    print(f"\nclang-tidy passed on {total} files.")
+
+
+if __name__ == "__main__":
+    main()