refactor(FN-1317): rename remaining kb references to fn

- Update executor, triage, and merger prompt strings to identify the product as "fn"
- Align branch/worktree documentation examples with fusion/fn-* naming conventions
- Rename dashboard websocket attachment guard from __kbWebSocketsAttached to __fnWebSocketsAttached
- Refresh engine tests, dashboard utility comments, and TUI header text to remove stale kb wording

packages/tui/src/index.tsx

@@ -1,4 +1,4 @@
-/** @fusion/tui — Terminal UI components for kb */
+/** @fusion/tui — Terminal UI components for fn */
 
 import { render, Text } from "ink";
 import React from "react";