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/engine/src/pi.ts

@@ -1,5 +1,5 @@
 /**
- * Shared pi SDK setup for kb engine agents.
+ * Shared pi SDK setup for fn engine agents.
  *
  * Uses the user's existing pi auth (API keys / OAuth from ~/.pi/agent/auth.json).
  * Provides factory functions for creating triage and executor agent sessions.
@@ -254,7 +254,7 @@ async function registerExtensionProviders(cwd: string, modelRegistry: ModelRegis
 }
 
 /**
- * Create a pi agent session configured for kb.
+ * Create a pi agent session configured for fn.
  * Reuses the user's existing pi auth and model configuration.
  */
 export async function createKbAgent(options: AgentOptions): Promise<AgentResult> {