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/agent-generation.ts

@@ -40,28 +40,13 @@ async function initPromptCatalog() {
 // Initialize prompt catalog (will be awaited in actual usage)
 const promptCatalogReadyPromise = initPromptCatalog();
 
-// Dynamic import for @fusion/engine to avoid resolution issues in test environment
+import { createFnAgent as engineCreateFnAgent } from "@fusion/engine";
+
 // eslint-disable-next-line @typescript-eslint/no-explicit-any
-let createFnAgent: any;
+let createFnAgent: any = engineCreateFnAgent;
 
-// Initialize the import (this runs in actual server, mocked in tests)
-async function initEngine() {
-  if (!createFnAgent) {
-    try {
-      const engineModule = "@fusion/engine";
-      const engine = await import(/* @vite-ignore */ engineModule);
-      createFnAgent = engine.createFnAgent;
-    } catch {
-      // Allow failure in test environments - agent functionality will be stubbed
-      createFnAgent = undefined;
-    }
-  }
-}
-
-let engineReady: Promise<void> | undefined;
-function ensureEngineReady() {
-  engineReady ??= initEngine();
-  return engineReady;
+function ensureEngineReady(): Promise<void> {
+  return Promise.resolve();
 }
 
 // ── Constants ───────────────────────────────────────────────────────────────