Add git worktree safety guidance to AGENTS.md

[vcolin7]

What does this PR do?

Documents a git stash hazard for contributors who work in linked git worktrees. When multiple worktrees share one repository common git directory, Git stores stash entries in the shared refs/stash, so stash operations from concurrent worktrees share a single stack. A stash created, popped, or dropped in one worktree can therefore apply or restore the wrong changes in another. This caused a real near-miss during parallel work, so this PR adds explicit guidance to steer contributors away from it.

The change is documentation-only and edits a single file, AGENTS.md:

• Adds a new Git Worktree Safety subsection under "Git Workflow and Automation" explaining the problem and recommending safer alternatives (git diff <ref>, a throwaway git worktree add of the base branch, and explicit staging with git add -p / specific paths instead of git add -A).
• Adds one concise bullet to the top-level "Don't" list.

Spelling was verified with .\eng\common\spelling\Invoke-Cspell.ps1 (passes). No build or tests are needed for a docs-only change.

GitHub issue number?

N/A

Pre-merge Checklist

• Required for All PRs
  • Read contribution guidelines
  • PR title clearly describes the change
  • Commit history is clean with descriptive messages (cleanup guide)
  • Added comprehensive tests for new/modified functionality (N/A - docs-only change)
  • Created a changelog entry if the change falls among the following: new feature, bug fix, UI/UX update, breaking change, or updated dependencies (N/A - internal contributor documentation only)
• For MCP tool changes: N/A - no tool changes in this PR

Invoking Livetests

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[Copilot]

Pull request overview

This PR updates contributor guidance in AGENTS.md to document a safety hazard when using git stash across multiple linked worktrees that share a common .git directory, and provides safer workflow alternatives to avoid accidentally applying/dropping the wrong changes.

Changes:

• Add a new “Git Worktree Safety” subsection under “Git Workflow and Automation” describing why git stash is unsafe with linked worktrees and suggesting alternatives.
• Add a new item to the top-level “Don’t” list warning against git stash in linked worktrees.

[JasonYeMSFT]

Is this issue surfaced in an agent session? For a human author, when they see the poped change, they should notice that the diff is not what they expect, right?