fix(filesystem): gracefully handle unavailable directories on startup

[Sean-Kenneth-Doherty]

Summary

Instead of failing entirely when any configured allowed directory is unavailable (e.g., unmounted network volume, disconnected external drive), the server now:

• Warns about each unavailable directory and skips it
• Continues starting with all available directories
• Only fails if NO directories are accessible

Changes

• Modified startup validation in src/filesystem/index.ts to collect unavailable directories and filter them out instead of calling process.exit(1) on the first failure
• Added clear warning messages showing which directories were skipped
• Server exits with error only when zero directories are accessible

Testing

• All 139 existing tests pass
• Manual testing confirms:
  • Server starts with mixed valid/invalid directories (warns and continues)
  • Server fails gracefully with all invalid directories (helpful error message)

Impact

This makes the filesystem MCP server more resilient for setups involving:

• Network shares/NAS devices
• External drives
• Mounted volumes
• Any path that might be temporarily unavailable

Fixes #3232