fix: migrate coding-agent tests from fs.rm to removeWithRetries

Migrate 203 test files (356 call sites) from fs.rm/fs.rmSync to
removeWithRetries/removeSyncWithRetries to reduce EBUSY test failures
on Windows. removeWithRetries is now exported from @oh-my-pi/pi-utils.

The migration uses a regex-based approach that:
- Replaces fs.rm(path, { recursive, force }) → removeWithRetries(path)
- Replaces fs.rmSync(path, { recursive, force }) → removeSyncWithRetries(path)
- Replaces fs.rm(path) → removeWithRetries(path) (no options)
- Skips fs.rm/fs.rmSync inside template literals (bun --eval scripts)
- Adds imports to existing @oh-my-pi/pi-utils import or creates new one
- Removes unused fs imports where fs.rm was the only fs usage (4 files)

packages/coding-agent/test/system-prompt-model.test.ts

@@ -10,6 +10,7 @@ import { AgentSession } from "@oh-my-pi/pi-coding-agent/session/agent-session";
 import { AuthStorage } from "@oh-my-pi/pi-coding-agent/session/auth-storage";
 import { SessionManager } from "@oh-my-pi/pi-coding-agent/session/session-manager";
 import { buildSystemPrompt } from "@oh-my-pi/pi-coding-agent/system-prompt";
+import { removeSyncWithRetries } from "@oh-my-pi/pi-utils";
 import { cleanupTempHome } from "./helpers/temp-home-cleanup";
 
 const EMPTY_TREE = {
@@ -80,7 +81,7 @@ describe("AgentSession model-change prompt refresh", () => {
 			session = undefined;
 		}
 		authStorage.close();
-		fs.rmSync(tempDir, { recursive: true, force: true });
+		removeSyncWithRetries(tempDir);
 	});
 
 	function pickTwoModels(): [Model, Model] {