Make Ghostty available to New Siri / Apple Intelligence

We already support AppIntents, but they are not yet discoverable by Apple Intelligence.

Here are my expectations for what should work when marking this issue as complete:

• Siri can search opened terminals
• User can select the list results in Siri and open that terminal
• Siri can do actions like creating/closing splits, tabs and windows

Most of the APIs are already provided by AppIntents, here are some reference docs:

• https://developer.apple.com/documentation/appintents/making-actions-and-content-discoverable-by-apple-intelligence
• https://developer.apple.com/videos/play/wwdc2026/344/