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

### 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](https://docs.meshery.io/project/contributing) and Resources
- 📚 [Instructions for contributing to documentation](https://docs.meshery.io/project/contributing/contributing-docs)
- Meshery documentation [site](https://docs.meshery.io/) and [source](https://github.com/meshery/meshery/tree/master/docs)
- 🛠 [Meshery Build & Release Strategy](https://docs.meshery.io/project/build-and-release)
- 🎨 Wireframes and [designs for Layer5 site](https://www.figma.com/file/5ZwEkSJwUPitURD59YHMEN/Layer5-Designs) in Figma [(open invite)](https://www.figma.com/team_invite/redeem/GvB8SudhEOoq3JOvoLaoMs)
- 🙋🏾🙋🏼 Questions: [Layer5 Discussion Forum](https://discuss.layer5.io) and [Layer5 Community Slack](http://slack.layer5.io)

Start by reading ui/package.json and inspecting the repository's existing setup to identify the documented build and run commands. Consult node_modules/@sistent/sistent/DESIGN.md or the pinned v0.15.12 release, then create the root AGENTS.md with concise project guidance and Sistent rules explicitly scoped to ui/; done means no duplicate DESIGN.md is added.

Written by the indexing model from the issue text.