Add terminal history capture for Ghostty

Shell Sage currently does not capture terminal context directly from Ghostty. Please add Ghostty screen-history capture to make ssage effective on Ghostty.

The issue names Ghostty screen-history capture but no files, tests, or entry points. Start by locating the existing terminal-context capture implementation and its tests, then trace how another terminal is supported. Done means Shell Sage captures the relevant Ghostty screen history and the behavior is covered by a test.

Written by the indexing model from the issue text.