[Docs] Add AGENTS.md with Sistent design guidance (scoped to UI)

[PARTH-TUSSLE]

Current State

The layer5io/kanvas-docker-extension repository does not currently have an AGENTS.md file.
The repository consumes @sistent/sistent@0.15.12 specifically within the ui/ directory (ui/package.json), but there is no repository-level guidance for contributors or AI coding agents covering the project setup or directing UI-related work to Sistent's design contract.

Desired State

Create a lightweight, beginner-friendly root-level AGENTS.md that provides:

1. Basic project development/runbook and build setup.
2. UI-specific guidelines scoped strictly to the ui/ directory.
3. A clear pointer to Sistent's design contract matching the pinned version.

Guidance Requirements

• Directory Scope: Explicitly limit Sistent UI rules to ui/.
• Version Parity: Point contributors and agents to node_modules/@sistent/sistent/DESIGN.md (or the pinned release tag https://github.com/layer5io/sistent/tree/v0.15.12/DESIGN.md) instead of Sistent's master branch to prevent version mismatch.
• Single Source of Truth: Direct contributors and AI agents to consult Sistent before modifying UI styling, avoiding any duplicate DESIGN.md inside this repo.

Scope

• Create a concise AGENTS.md at the repository root.
• Document basic build and run commands for the project.
• Add the Sistent pointer under a UI section scoped to ui/.
• Keep this initial AGENTS.md compact and easy to pick up.

Contributor Guides and Resources

• 📚 Instructions for contributing to documentation
  • Meshery documentation site and source
• 🛠 Meshery Build & Release Strategy
• 🎨 Wireframes and designs for Layer5 site in Figma (open invite)
• 🙋🏾🙋🏼 Questions: Layer5 Discussion Forum and Layer5 Community Slack

[PARTH-TUSSLE]

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