fix(engine): make pi.js imports static to fail fast on partial dist

Replace lazy `await import("./pi.js")` and `require("./pi.js")` calls in
runtime-resolution, agent-session-helpers, agent-heartbeat, and
cron-runner with top-level static imports. These dynamic imports were
documented as plugin-decoupling, but pi.js is already eagerly loaded
through index.ts re-exports and static imports in executor/merger/
reviewer/triage/mission-execution-loop, so the deferral never paid off
in practice.

The deferral did, however, introduce a TOCTOU race: a tsc rebuild that
momentarily emptied dist/pi.js would let the engine load fine and only
fail minutes later when the first session was created (e.g. FN-2860
errored two minutes into execution while pi.js was being rewritten).
With static imports, a missing/half-built dist now fails immediately at
process startup with a clear stack — verified by `mv dist/pi.js
dist/pi.js.bak` reproducing ERR_MODULE_NOT_FOUND on the first import of
runtime-resolution.js.

Also drops the DefaultPiRuntime.describeModelFn cache, which only
existed to paper over the require-on-first-call latency.

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

packages/engine/src/cron-runner.ts

@@ -5,6 +5,7 @@ import type { AutomationStore } from "@fusion/core";
 import type { ScheduledTask, AutomationRunResult, AutomationStep, AutomationStepResult, Column, TaskCreateInput } from "@fusion/core";
 import { createLogger } from "./logger.js";
 import { defaultShell } from "./shell-utils.js";
+import { createFnAgent, promptWithFallback } from "./pi.js";
 
 const execAsync = promisify(exec);
 const log = createLogger("cron-runner");
@@ -628,9 +629,6 @@ const AI_AUTOMATION_SYSTEM_PROMPT = [
  * @returns An AiPromptExecutor function suitable for CronRunnerOptions.
  */
 export async function createAiPromptExecutor(cwd: string): Promise<AiPromptExecutor> {
-  // We import lazily to keep the factory self-contained and to avoid
-  // pulling pi.ts into the module graph when AI execution isn't used.
-  const { createFnAgent, promptWithFallback } = await import("./pi.js");
   const disposeLog = createLogger("cron-runner");
 
   return async (prompt: string, modelProvider?: string, modelId?: string): Promise<string> => {