Add folder size check workflow to prevent oversized directories

[Miyamura80]

Summary

• Folder-size CI workflow (.github/workflows/folder-size.yaml): runs on PRs touching .py files, checks that no single directory has too many flat Python siblings (warn at 20, error at 35).
• Large-file CI workflow (.github/workflows/large-files.yaml): runs on PRs touching .py files, checks that no source file exceeds the line-count threshold (warn at 500, error at 800).
• Shared bash scripts (scripts/check_folder_sizes.sh, scripts/check_large_files.sh): reusable scripts consumed by both GitHub Actions and local prek hooks. Support --all mode and per-file mode, produce GitHub step summaries, and respect grandfathered/excluded paths.
• Prek hooks (prek.toml): added check-large-files and check-folder-sizes hooks so the same checks run locally on every commit.
• Folder-refactor-advisor subagent (.claude/agents/folder-refactor-advisor.md, .codex/agents/folder-refactor-advisor.toml): advisory-only agent that diagnoses oversized folders and proposes three concrete subfolder restructurings when the CI check fails.
• CLAUDE.md updates: added prek install instructions at the top and a pointer to the folder-refactor-advisor subagent.

Test plan

• Push a PR with a folder containing >35 .py files and verify the folder-size workflow fails
• Push a PR with a .py file >800 lines and verify the large-files workflow fails
• Run scripts/check_folder_sizes.sh --all and scripts/check_large_files.sh --all locally and confirm clean output
• Verify prek hooks fire on git commit for staged .py files

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