feat: add foundation CesiumJS skills (spatial-math, core-utilities, time-properties)

[jdehorty]

Summary

Adds the first 3 of 13 CesiumJS domain-level skills, covering the foundational API surface that all other domains depend on. Modeled after CloudAI-X/threejs-skills.

• cesiumjs-spatial-math (~13.5KB, 351 lines) -- Cartesian3, Cartographic, Matrix4, Quaternion, Transforms, Ellipsoid, BoundingSphere, projections, coordinate conversions
• cesiumjs-core-utilities (~13KB, 403 lines) -- Resource, Color, Event, RequestScheduler, error handling, PinBuilder, helper functions
• cesiumjs-time-properties (~15KB, 353 lines) -- Clock, JulianDate, Property system (SampledProperty, CallbackProperty, etc.), splines, interpolation

Also adds:

• skills/cesiumjs/DOMAINS.md -- master domain-to-class mapping (source of truth for all 13 planned domains)
• Updated skills/README.md -- documents the new skills and the complementary relationship between baked-in skills and cesium-context7

Each skill follows the threejs-skills template: minimal frontmatter, Quick Start, API reference sections, runnable ES module code examples, Performance Tips, and See Also cross-references. Based on CesiumJS v1.139.x.

This is PR 1 of 4 in a stacked series that will add all 13 CesiumJS skills:

1. Foundation (this PR) -- spatial-math, core-utilities, time-properties
2. Core Rendering -- viewer-setup, imagery, terrain-environment, materials-shaders
3. Content -- entities, primitives, 3d-tiles, camera
4. Interaction -- interaction, models-particles

Test plan

• Verify each SKILL.md has correct frontmatter (name and description fields)
• Verify code examples use ES module imports and are self-contained
• Verify no real API keys or credentials (all should be placeholders like YOUR_TOKEN)
• Verify DOMAINS.md class listings match the CesiumJS v1.139 public API
• Verify See Also cross-references point to valid skill names

[MKartaviciute]

I think this one should go to new repo cesiumjs-skills.

[MKartaviciute]

After we created a separate repository https://github.com/CesiumGS/cesiumjs-skills, need to reference this new repository in the README