btversion: exclude the game-rewritten content/LAST.EGG from the dirty check

The game rewrites LAST.EGG at every run, so every build stamped '+'
(false-dirty) even from a pristine checkout.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

tools/btversion.cmake

@@ -15,7 +15,10 @@ execute_process(COMMAND git rev-parse --short HEAD
                 WORKING_DIRECTORY "${SRC}"
                 OUTPUT_VARIABLE BT_HASH OUTPUT_STRIP_TRAILING_WHITESPACE
                 ERROR_QUIET)
+# content/LAST.EGG is rewritten by the game at every run -- a permanent
+# false-dirty; exclude it so a pristine checkout stamps clean.
 execute_process(COMMAND git status --porcelain --untracked-files=no
+                        -- . ":(exclude)content/LAST.EGG"
                 WORKING_DIRECTORY "${SRC}"
                 OUTPUT_VARIABLE BT_DIRTY_RAW
                 ERROR_QUIET)