fix(dashboard,core,engine): statically import @fusion/engine to fix createFnAgent undefined in published CLI

The dashboard modules used a variable-specifier dynamic import
(`const m = "@fusion/engine"; await import(m)`) to defeat bundler static
analysis. tsup honored that and left the dynamic import in dist/bin.js,
so the published `@runfusion/fusion` package failed at runtime with
"createFnAgent2 is not a function" — `@fusion/engine` isn't on npm and
the silent catch set the binding to undefined. Replaces the trick with
static imports across planning, chat, subtask-breakdown, mission-interview,
agent-generation, ai-refine, roadmap-suggestions, milestone-slice-interview,
and routes. Core can't statically import engine (cycle), so it now exposes
setCreateFnAgent and engine wires itself in at module load. Documents the
pattern in AGENTS.md.

Fixes Runfusion/Fusion#9.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

packages/dashboard/src/subtask-breakdown.ts

@@ -9,9 +9,10 @@ import {
   resetDiagnosticsSink,
 } from "./ai-session-diagnostics.js";
 
+import { createFnAgent as engineCreateFnAgent } from "@fusion/engine";
+
 // eslint-disable-next-line @typescript-eslint/no-explicit-any
-let createFnAgent: any;
-const engineModule = "@fusion/engine";
+let createFnAgent: any = engineCreateFnAgent;
 
 /**
  * Shared diagnostics helper for the subtask-breakdown module.
@@ -45,21 +46,8 @@ export function __setSubtaskBreakdownDiagnostics(_logger: unknown): void {
   }
 }
 
-async function initEngine() {
-  if (!createFnAgent) {
-    try {
-      const engine = await import(/* @vite-ignore */ engineModule);
-      createFnAgent = engine.createFnAgent;
-    } catch {
-      createFnAgent = undefined;
-    }
-  }
-}
-
-let engineReady: Promise<void> | undefined;
-function ensureEngineReady() {
-  engineReady ??= initEngine();
-  return engineReady;
+function ensureEngineReady(): Promise<void> {
+  return Promise.resolve();
 }
 
 export interface SubtaskItem {