FRE gives no success confirmation after a clean save

### Description of the new feature

The FRE surface only speaks up when something goes wrong. After a clean Save, where settings were written, hooks installed, and shell integration set up, there is no positive signal that any of it actually turned on. The user is dropped into the terminal and has to infer that it worked.

Add a brief success confirmation on completion, naming what is now on. For example: "Error detection is enabled" and whichever other capabilities were configured.

### Proposed technical implementation details

Needs design input on the surface: inline on the last FRE page before dismissal, a transient toast after the terminal opens, or a first-run banner in the agent pane.

Related: #231 reports the inverse problem, where a user could not tell whether setup had completed or hung.