fix(pi-fff): cache sdk import across reloads to avoid /reload hang

[chenydev]

Closes #757

Root cause

Pi reloads extension modules with jiti moduleCache: false, so loadSdk() in packages/pi-fff/src/sdk.ts re-executes a dynamic import("@ff-labs/fff-bun") on every /reload. The fff-bun module graph top-level awaits a type: "file" import of the native libfff_c.so (packages/fff-bun/src/embedded.ts), which never resolves when re-imported inside the Bun-compiled pi binary. Since pi awaits all session_start handlers without a timeout, the reload screen sticks on "Reloading keybindings..." forever (CPU ≈ 0).

The first import at startup resolves fine (~20ms); plain bun re-imports also work — the hang is specific to the pi Bun binary + jiti combination.

Fix

Cache the first import on globalThis so reloads reuse the resolved module instead of re-importing:

const g = globalThis as Record<string, unknown>;
if (g.__fffSdkPromiseGlobal) {
  sdkPromise = g.__fffSdkPromiseGlobal as Promise<{ FileFinder: FileFinderStatic }>;
  return sdkPromise;
}
...
sdkPromise = p;
(globalThis as Record<string, unknown>).__fffSdkPromiseGlobal = p;

Verification

Reproduced in an isolated env (copied agent dir + PI_CODING_AGENT_DIR, PTY driver):

• Before: /reload stuck >60s, no "Reloaded keybindings" line, process at ~0% CPU.
• After: 3 consecutive /reload calls all succeed in ~2s with the full 28-package user settings.

Note: #599 (non-blocking session_start warmup) would mask the UI hang, but the background import would still hang and break subsequent fff tool calls, which await the same in-flight sdkPromise. This import-level fix is complementary.