feat: add cesium mcp app skill

[MKartaviciute]

Description

This pull request introduces a new agent skill, cesium-mcp-app, to the Cesium ecosystem skills directory. The skill provides comprehensive, step-by-step guidance for building Cesium MCP Apps that expose CesiumJS visualizations as interactive tool suites for LLMs. The changes include detailed documentation, workflow guides, CSP and build configuration references, and evaluation instructions to ensure production-ready MCP Apps. The most important changes are grouped by theme below.

Addition of the cesium-mcp-app skill:

• Added cesium-mcp-app to the skills directory and updated skills/README.md to describe its purpose and input types.
• Created a comprehensive README.md for cesium-mcp-app detailing its workflow, valid inputs, required files, prerequisites, and related skills.

Workflow and implementation guidance:

• Added SKILL.md with step-by-step instructions for building, modifying, and evaluating Cesium MCP Apps, including input handling, API verification, tool design, implementation, CSP configuration, and evaluation phases.

Reference and configuration documentation:

• Added reference/csp-config.md outlining required CSP domains, build configuration templates (vite.config.ts, tsconfig.json, package.json), and dependency installation for Cesium MCP Apps.

Evaluation guide and harness instructions:

• Added reference/evaluation.md providing a detailed guide for designing, running, and interpreting scenario-based evaluations for Cesium MCP Apps, including harness usage and scoring methodology.

Issue number and link

Testing plan

Can be used as a slash command: /cesium-mcp-app use https://sandcastle.cesium.com/?id=imodel-mesh-export-service

Author checklist

• I have submitted a Contributor License Agreement
• I have performed a self-review of my code