feat(tui): notify when a session enters the waiting state

[jongio]

Summary

Adds a notify_on_waiting setting that rings the terminal bell and shows a short footer message when a session enters the waiting state (the AI has responded and is waiting for user input).

Closes #209

Behavior

• Fires once per transition into waiting, not on every scan.
• Sessions already waiting when dispatch starts do not trigger the bell (the first scan only records a baseline).
• A session that leaves and later re-enters waiting notifies again.
• When several sessions enter waiting in the same scan, the bell rings once and the footer reads "N sessions are waiting".
• Off by default. Toggle it from the Settings panel (,) as "Notify On Waiting", or set "notify_on_waiting": true in config.json.

Changes

• internal/config/config.go: new NotifyOnWaiting bool field.
• internal/tui/handlers.go: bell command with a swappable seam, plus transition detection in handleAttentionScanned.
• internal/tui/model.go, internal/tui/cmdpalette.go: wire the setting through the Settings panel load/save paths.
• internal/tui/components/configpanel.go: new "Notify On Waiting" toggle row.
• README: documented the option, updated the example config and Settings panel field list.

Testing

• go build ./...
• go vet ./...
• golangci-lint run ./... (0 issues)
• go test ./... -count=1 (all packages pass)
• New tests cover baseline suppression, transition, disabled setting, steady state, re-entry, multiple sessions, forgotten sessions, the bell command, and the handler wiring.