fix(core): convert AsyncOperation to UniTask for Unity 6 compatibility

[JasonXuDeveloper]

Summary

• Fix CS0311 compilation error in Unity 6 where UnityEngine.AsyncOperation cannot be directly awaited
• Convert SceneManager.LoadSceneAsync() to use .ToUniTask() extension method
• Add git workflow guidelines (branch from master, use PRs for all changes)

Problem

In Unity 6, UnityEngine.AsyncOperation cannot be used as a type parameter for EnumeratorAsyncExtensions.GetAwaiter<T>() because there's no implicit reference conversion to System.Collections.IEnumerator.

Solution

Use the ToUniTask() extension method from Cysharp.Threading.Tasks to convert the AsyncOperation to a UniTask, which can be awaited. This pattern is already used elsewhere in the codebase (e.g., line 145 in the same file).

Test plan

• Open the project in Unity 6
• Verify no CS0311 compilation errors
• Test scene loading error path to ensure fallback scene loads correctly

🤖 Generated with Claude Code

[Copilot]

Pull request overview

This PR fixes a Unity 6 compilation error where UnityEngine.AsyncOperation cannot be directly awaited due to missing implicit reference conversion to System.Collections.IEnumerator. The fix uses the .ToUniTask() extension method, which is the established pattern already used elsewhere in the same file. Additionally, the PR adds git workflow guidelines to ensure proper branching practices and PR-based development workflow.

Changes:

• Fixed CS0311 compilation error by converting SceneManager.LoadSceneAsync() to use .ToUniTask() in the error handling path
• Added comprehensive git workflow guidelines covering branching from master and PR-based workflow requirements

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
UnityProject/Packages/com.jasonxudeveloper.jengine.core/Runtime/Bootstrap.Common.cs	Converted AsyncOperation to UniTask using .ToUniTask() to fix Unity 6 compilation error
.claude/rules/commit-conventions.md	Added git workflow sections for branching from master and PR-only workflow requirements

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[github-actions]

Unity Test Results

✅ EditMode: All tests passed
✅ PlayMode: All tests passed

Unity Version: 2022.3.55f1
Project Path: UnityProject

✅ All tests passed! The PR is ready for review.

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[github-actions]

Auto-approved: Copilot review found no issues and Unity Tests passed (or were skipped for non-code changes).