bash: add license declaration for kitty-derived code (#4776)

src/shell-integration/bash/ghostty.bash

@@ -1,6 +1,19 @@
-# This is originally based on the recommended bash integration from
-# the semantic prompts proposal as well as some logic from Kitty's
-# bash integration.
+# Parts of this script are based on Kitty's bash integration. Kitty is
+# distributed under GPLv3, so this file is also distributed under GPLv3.
+# The license header is reproduced below:
+#
+# This program is free software: you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
 
 # We need to be in interactive mode and we need to have the Ghostty
 # resources dir set which also tells us we're running in Ghostty.