chore: use pre-commit for clang-format - #499

Problem

EloqStore formatting currently depends on a repository script that downloads a
full LLVM 18.1.8 archive, installs libtinfo5, and creates a system-wide binary.
Local formatting and CI also define their file scopes independently.

Observable behavior

Developers can now install the hook with pre-commit install and format the
tracked tree with pre-commit run --all-files. Pre-commit downloads and caches
its isolated clang-format 18.1.8 environment without modifying the system LLVM
installation.

Implementation

• Add a pre-commit configuration pinned to mirrors-clang-format v18.1.8.
• Format project-owned C/C++ sources while excluding external/ and the Rust
  vendor-link tree.
• Run the same pre-commit command in the clang-format CI workflow.
• Remove the manual clang-format installation/formatting script.
• Update README.md and CLAUDE.md with the new setup and formatting commands.

Design decisions

The exclusions preserve the existing policy of not reformatting third-party
sources. The Rust vendor tree is excluded explicitly because its source and
include entries link back to repository-root trees and should not be scanned a
second time. Build directories need no explicit exclusion because --all-files
only passes Git-tracked files to hooks.

Verification

• pre-commit validate-config
• pre-commit run --all-files --show-diff-on-failure
• Confirmed the hook-installed binary reports clang-format 18.1.8.
• Confirmed representative paths under both excluded trees report no files to
  check, while a project source file runs through the hook.
• Parsed .github/workflows/Clang-format-checker.yml with PyYAML.
• git diff --check origin/main...HEAD

Full C++ build and ctest --test-dir build/tests/ were not run because this PR
does not modify runtime or test code.

Risk and rollback

The change only affects developer and CI tooling. Its main operational risk is
that the first pre-commit run needs network access to populate the hook cache.
Rollback is limited to restoring the previous CI action and formatting script.

Reviewer focus

Please review the path exclusions in .pre-commit-config.yaml and confirm the
CI workflow should use that configuration as the single formatting definition.

Checklist

• Add tests for the change (not applicable: tooling-only change)
• Document changes
• Reference issue (none)
• Reference RFC (none)
• Pass ctest --test-dir build/tests/ (not run: no runtime code changes)

Summary by CodeRabbit

• Chores

  • Updated code-formatting checks to use a pinned pre-commit configuration with clang-format 18.1.8.
  • Formatting checks now consistently run locally and in CI, with external and vendored directories excluded.
  • Added the pre-commit configuration to version control.

• Documentation

  • Updated setup and formatting instructions in the README and contributor guidance.
  • Documented installing the hook, formatting all files, and bypassing checks when necessary.

• Removed

  • Removed the standalone formatting script.