TUI — worker toast notifications

A small stack of status lines appears between the content area and the key-hint footer whenever a worker finishes or fails — so a state change is noticed without watching rows.

A toast fires the moment a worker transitions from an active state (pending or running) to a terminal state:

Transition	Glyph	Color
-> done	✔	green
-> error	✕	red
-> cancelled	⊘	yellow

The label reads <provider or agentId> <state> — <workflow name>. Workers already terminal when the TUI starts do not toast (no flood on attach). Each toast lives ~4 seconds then disappears; at most 3 show at once (older ones drop); when none are active the footer layout is unchanged.