feat: add AGENT.md documentation for Stacks Wars backend architecture

[iflames1]

No description provided.

[cursor]

Bugbot is not enabled for your account, so this pull request was not reviewed.

Enable Bugbot in the Cursor dashboard to get automatic reviews on future PRs.

[Copilot]

🟡 Not ready to approve

The new documentation contains at least one incorrect repository path (migrations) and a potentially ambiguous “Don’t” guideline that should be clarified to avoid misleading contributors.

Once you've addressed the issues Copilot identified, you can request another Copilot review.

This review doesn't count toward merge requirements. Sign up for the private preview to control whether Copilot approvals count.

Pull request overview

Adds a new AGENT.md document to serve as canonical contributor/agent guidance for the Stacks Wars backend architecture, with emphasis on crate boundaries and platform/game-engine contracts.

Changes:

• Introduces workspace/crate ownership and architectural boundary rules (sw-domain, sw-plugin, sw-server).
• Documents runtime expectations for WebSocket messaging, vault/transaction flow, and database/migrations.
• Adds “Do/Don’t” guidance for game engine integration and security hygiene.

File summaries

File	Description
AGENT.md	New architecture/contributor guidance covering crate boundaries, WS patterns, vault/transactions, DB/migrations, and game registration.

Review details

• Files reviewed: 1/1 changed files
• Comments generated: 2
• Review effort level: Lite

We're testing this review assessment. Please use 👍 or 👎 to tell us if it's correct.