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/plugin-install-validation.test.ts

@@ -4,6 +4,7 @@ import * as os from "node:os";
 import * as path from "node:path";
 import { PluginManager } from "@oh-my-pi/pi-coding-agent/extensibility/plugins/manager";
 import * as piUtils from "@oh-my-pi/pi-utils";
+import { removeWithRetries } from "@oh-my-pi/pi-utils";
 import type { Subprocess } from "bun";
 
 function emptyStream(): ReadableStream<Uint8Array> {
@@ -64,7 +65,7 @@ describe("PluginManager.install load validation", () => {
 
 	afterEach(async () => {
 		vi.restoreAllMocks();
-		await fs.rm(tmpRoot, { recursive: true, force: true });
+		await removeWithRetries(tmpRoot);
 	});
 
 	test("rejects an install whose extension entry cannot resolve its dependencies", async () => {