docs: add NextPress AI agent skills (theme, plugin, core)

[AminMemariani]

What

Adds a docs/skills/ folder containing source-grounded AI agent skills for the three NextPress contribution tracks:

• theme-development — layouts, WordPress-style template hierarchy, block render overrides, theme.json.
• plugin-development — PluginContext surface, hook events (actions + filters), field/content-type/block registration.
• core-development — engine guardrails (dependency direction, framework-free core, siteId scoping, permission guards, sanitization), tRPC/service/Prisma patterns.

Each skill is a folder with SKILL.md (rules + workflow) and reference.md (exact API signatures with file:line citations), following the Agent Skills convention. Plus a docs/skills/README.md index.

Why

Gives contributors and AI coding agents an accurate, progressive-disclosure guide per track, extracted directly from the real source (types, resolver logic, hook names, service patterns) rather than prose.

Scope

Documentation only — markdown files under docs/skills/. No source, schema, or config changes; cannot affect typecheck or tests.

⛩️ Generated with ehAye Dojo 🚀