gitignore macOS entitlements and dugite binary, add Linux picker handler

DebugProfile.entitlements gitignored (machine-specific sandbox paths).
native/dugite/ gitignored (downloaded at build time).

Linux runner: add pickDirectory handler using GtkFileChooserDialog
in the existing clide/window method channel.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

.gitignore

@@ -1,3 +1,6 @@
+# -- macOS sandbox (machine-specific paths; template is committed) ------
+/macos/Runner/DebugProfile.entitlements
+
 # -- Flutter / Dart (single package at repo root) -----------------------
 /.dart_tool/
 /.packages
@@ -27,6 +30,9 @@ tools/ui/.serve.pid
 /ptyc/bin/
 /ptyc/*.o
 
+# -- dugite-native (bundled git, downloaded at build time) ---------------
+/native/dugite/
+
 # -- Test, coverage, profile output ------------------------------------
 *.test
 *.out