feat: add case-insensitive filesystem guard hook

[yurukusa]

• Adds case-sensitive-guard.js — detects case-insensitive filesystems (exFAT, NTFS, HFS+) and blocks rm/mkdir collisions
• Inspired by anthropics/claude-code#37875 where rm -rf content on exFAT destroyed data because Content and content resolved to the same path

1. Extracts target path from mkdir/rm commands
2. Creates a temp file to probe if the filesystem is case-insensitive
3. Scans parent directory for case-variant entries
4. Exits 2 (block) if collision detected

• node -c syntax check passes
• Follows existing hook patterns (stdin JSON, exit codes)
• Manual test on case-insensitive filesystem
  🤖 Generated with Claude Code