feat: Add GitHub App authentication support

[shit-agents]

Summary

Native GitHub App authentication for bot-attributed commits and higher rate limits.

New Features

Static Factory Methods

// Using config-based credentials
$github = Github::forInstallation(98848199);

// Using explicit credentials  
$github = Github::withApp($appId, $privateKey, $installationId);

InstallationsResource

$github->installations()->all();           // List all installations
$github->installations()->get($id);        // Get specific installation
$github->installations()->getToken($id);   // Get access token (cached 55min)

Config

'github_app' => [
    'app_id' => env('GITHUB_APP_ID'),
    'private_key' => env('GITHUB_APP_PRIVATE_KEY'),
    'private_key_path' => env('GITHUB_APP_PRIVATE_KEY_PATH'),
    'installation_id' => env('GITHUB_APP_INSTALLATION_ID'),
],

Files Added (869 lines)

• src/Resources/InstallationsResource.php
• src/Data/Installations/InstallationData.php
• src/Data/Installations/InstallationTokenData.php
• src/Requests/Installations/*.php
• tests/Feature/GitHubAppAuthenticationTest.php

Why GitHub Apps?

• Higher rate limits (5,000/hour per installation vs 5,000/hour per user)
• Bot identity for commits/PRs (shows as app-name[bot])
• Granular repository permissions
• No personal access tokens needed

Closes #106

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Co-Authored-By: github-app-architect github-app-architect@the-shit.bot

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