add toast notification system (T-50)

Non-modal operation-feedback toasts, bottom-right: a ClideToast card per
severity (success/warning/error/info), auto-dismiss (errors linger), queue
with a visible cap, slide+fade in, manual dismiss, live-region a11y.

ToastService is a MessageBus consumer — components raise a toast by publishing
to the 'toast' channel (publishToast helper), so emitters stay decoupled from
the UI. GitController's push/pull are the first emitters. ToastOverlay mounts
in the app-root Stack.

Also adds comprehensive GitController coverage: importing it for the toast
emitter test first pulled the whole file into the coverage denominator, so the
controller is now tested end to end (status/stage/commit/stash/push/pull).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

CHANGELOG.md

@@ -18,6 +18,11 @@ heading, and (b) bumping `pubspec.yaml` `version:` in the same commit.
 
 ### Added
 
+- Toast notifications for operation feedback: non-modal cards slide in
+  bottom-right, auto-dismiss (errors linger), stack, and are manually
+  dismissable, with success/warning/error/info severities. Components raise
+  them by publishing to the kernel MessageBus — git push/pull show the first
+  ones. (T-50)
 - Claude pane folds runs of tool calls/results into a collapsible "activity
   card" so prose isn't buried: collapsed by default with a live one-line ticker
   + step count, click/Enter to expand. Claude prose, user messages, and failed