Add stackable toast notifications

Replaces the single-slot status-line flash with a top-right toast
stack over the focused pane. flashError, flashTransient, and
notifyAttention all push onto the same stack (cap 5, FIFO drop).
Ctrl-N dismisses the most recent toast; empty stack falls through to
the focused PTY so readline / nano / emacs / opencode bindings keep
working. A new "Clear notifications" palette item empties the stack.

CHANGELOG.md

@@ -6,6 +6,18 @@ loosely follows [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
 ## [Unreleased]
 
+### Changed
+- Replaced the single-slot status-line "flash" with a stackable toast
+  surface anchored at the top-right of the focused pane. `flashError`,
+  `flashTransient`, and MCP `request_human_attention` now push onto
+  the toast stack (cap 5, oldest drops). Toasts persist until
+  dismissed with `Ctrl-N`, or cleared via the new
+  "Clear notifications" palette command. The status line no longer
+  shows the `[!]` prefix.
+- `Ctrl-N` is consumed by the host only when there is a toast to
+  dismiss; an empty stack lets `Ctrl-N` pass through to the focused
+  child so readline / nano / emacs / opencode keep their bindings.
+
 ## [0.0.4] - 2026-05-15
 
 ### Changed