feature: Supabase skills

[Rodriguespn]

What kind of change does this PR introduce?

Introduces the foundational structure for the supabase agent skill - a comprehensive guide for AI agents working with Supabase products.

What is the current behavior?

There is no skill available to guide AI agents on how to properly use Supabase products.

What is the new behavior?

This PR adds the supabase skill scaffolding that will serve as a comprehensive guide covering Supabase core products: Auth, Database, Storage, Edge Functions, Realtime, CLI, supabase-js SDK, and MCP server.

Files Added

File	Description
SKILL.md	Agent-facing skill manifest following the Agent Skills Open Standard. Contains the skill metadata, description, and resource index pointing to all available references.
AGENTS.md	Auto-generated compiled document containing all references. Do not edit directly - regenerated by npm run build.
GETTING_STARTED.md	Step-by-step contribution guide for product teams to add their content.
README.md	Contributor guide with quick start commands and skill structure overview.
references/_template.md	Template file for creating new reference documents. Copy this when adding new content.

How to Contribute

To add content for your product, follow the instructions in GETTING_STARTED.md.

Quick summary:

1. Create a branch from feature/supabase-skill
2. Copy references/_template.md to references/{your-product}.md
3. Add your content following the template structure
4. Update the resource table in SKILL.md
5. Run npm run validate && npm run build && npm run check
6. Open a PR targeting feature/supabase-skill