feat: open specific guild vaults by number

[darbyjack]

Summary

Add direct access to numbered guild vaults while preserving the existing vault picker behavior.

Players can now use:

• /guild vault to open the existing picker GUI
• /guild vault <number> to open a specific unlocked vault directly

Changes

• add an optional vault number argument to the existing vault command
• extract vault opening into a shared VaultGUI.open path used by both the command and picker
• validate one-based vault numbers against the guild tier
• create every missing cached vault up to the requested number before opening it
• use the loop index for picker numbering and click handling
• remove the shared companion-object counter from VaultGUI

Why

The previous picker kept a static counter shared by every GUI instance and only created one missing vault per click. Opening a higher-numbered vault with an incomplete cache could still result in an out-of-bounds lookup.

Local validation

Ran standalone Kotlin behavior checks covering:

• direct access to an unlocked vault
• filling all missing vaults up to the requested number
• opening the correct requested inventory
• rejecting vault zero
• rejecting vaults above the guild tier limit
• avoiding cache mutation for invalid requests

Fixes #323