Notify user in TUI when browser review is waiting for input

## Problem

When `plannotator_submit_plan` opens the browser for plan review, there is no TUI notification. The tool silently awaits the user decision, making pi appear frozen.

On GNOME (Wayland), `xdg-open` does not force the browser to the foreground either, so the user has no visual cue that anything happened.

## Flow

```
agent calls plannotator_submit_plan
→ server starts
→ browser opens (background, no TUI message)
→ pi awaits decision ← silent, appears stuck
→ user eventually notices browser tab and clicks Approve/Deny
```

## Suggested fix

In `openBrowserForServer`, when the browser opens successfully on a local session, notify the user:

```ts
} else {
ctx.ui.notify("Plannotator: browser opened — review and approve/deny when ready.", "info");
}
```

This ensures the TUI always communicates that plannotator is waiting on user input, regardless of whether the browser lands in the foreground.