fix: show path in external_directory permission prompt

[jamesbornholt]

Without this, external_directory permissions prompts, at least for read tools, didn't show a path at all. You could figure it out by seeing which tool usages were pending, but that was annoying especially if it was a different session triggering the prompt. The "always allow" confirmation screen did include the path though.

• Update bash tool to ask for each external directory individually (consistent with read/edit/patch tools)
• Populate metadata.parentDir in bash tool's external_directory request
• Display absolute path for external directories instead of relative