Expose terminal title as public variable - #656

Summary

• rename the buffer-local terminal title state from ghostel--title to public ghostel-title
• document the public variable and update core, OSC, picker, render, and consult tests
• add coverage confirming terminal titles remain buffer-local

Testing

• byte-compiled the changed package files
• ran the relevant Elisp and native buffer-name, buffer-picker, OSC, and render test suites
• ran make test-consult
• ran targeted checkdoc and docquote checks