feat: support CLAUDE_CONFIG_DIR environment variable

[ramarivera]

Summary

• Adds support for CLAUDE_CONFIG_DIR environment variable
• Falls back to ~/.config/claude (new Claude Code default) then ~/.claude (legacy)
• Fixes ENOTDIR error when ~/.claude is a file instead of directory

Fixes #6551

Changes

• Added CLAUDE_CONFIG_DIR to Flag namespace
• Added Global.claudeConfigDir() function with proper fallback logic
• Updated system.ts to use dynamic path for CLAUDE.md
• Updated skill.ts to use dynamic path for skills directory
• Added unit tests for the new logic

Testing

• Added unit tests covering all fallback scenarios (8 tests, all passing)
• Manually tested with CLAUDE_CONFIG_DIR set to custom path