[Repo Assist] fix: replace deprecated async fs.exists with fs.existsSync in addRootPath

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Summary

Replaces the deprecated async fs.exists() callback with the synchronous fs.existsSync() in addRootPath().

Root Cause

addRootPath() is called from loadSettings() to resolve relative executablePath values (e.g. ./vendor/bin/phpcbf) to absolute paths. The old code used fs.exists(path, callback) — a deprecated Node.js API that runs the existence check asynchronously:

// Before: async — the callback fires AFTER loadSettings() returns
fs.exists(tmpExecutablePath, exists => {
    if (exists) {
        this.executablePath = tmpExecutablePath;
    }
});

When loadSettings() returns, this.executablePath is still the original relative path. format() immediately calls getArgs() with that unresolved path, so cp.spawn() receives ./vendor/bin/phpcbf instead of the absolute path, causing a spawn failure.

This is the race condition behind issue #36: "after restart vscode it can't find the exePath anymore." The extension works after the second save (because the async callback has already fired by then), but fails on the first save after each restart.

Fix

// After: synchronous — executablePath is updated before loadSettings() returns
if (fs.existsSync(tmpExecutablePath)) {
    this.executablePath = tmpExecutablePath;
}

fs.existsSync() is synchronous, stable, and the idiomatic replacement for fs.exists() which has been deprecated since Node.js v4.

Files Changed

File	Change
extension.js	Replace fs.exists(path, callback) with if (fs.existsSync(path)) in addRootPath()

Test Status

This extension requires a live VS Code instance. No CI is configured for this repository.

Manual test steps:

1. Set phpcbf.executablePath to a relative path (e.g. ./vendor/bin/phpcbf)
2. Restart VS Code
3. Open a PHP file and save immediately — phpcbf should resolve and run correctly on the first save (was: failed until second save)

Closes #36

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