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/engine/src/index.ts

@@ -18,6 +18,13 @@ export { MissionExecutionLoop, type MissionExecutionLoopOptions, type Validation
 export { aiMergeTask, type MergerOptions } from "./merger.js";
 export { reviewStep, type ReviewType, type ReviewVerdict, type ReviewResult, type ReviewOptions } from "./reviewer.js";
 export { createFnAgent, promptWithFallback, describeModel, setHostExtensionPaths, getHostExtensionPaths, type AgentOptions, type AgentResult } from "./pi.js";
+
+// Register createFnAgent into core's loader so consumers in @fusion/core
+// (e.g. ai-summarize, memory-compaction) can resolve it without a circular
+// static import. Runs once at engine module load.
+import { setCreateFnAgent } from "@fusion/core";
+import { createFnAgent as _createFnAgentForCore } from "./pi.js";
+setCreateFnAgent(_createFnAgentForCore);
 export {
   resolveSessionSkills,
   createSkillsOverrideFromSelection,