feat: supabase skill with db and realtime references

Adds the supabase agent skill with comprehensive references for:
- Database: schema design, RLS policies, migrations, indexing, query optimization, security
- Realtime: channels, broadcast, presence, postgres changes, auth setup, error handling

GETTING_STARTED.md

@@ -0,0 +1,69 @@
+# Getting Started
+
+Contributor guide for adding content to the Supabase Agent Skills.
+
+## Quick Start
+
+1. Create a reference file in `skills/supabase/references/`
+2. Use `skills/supabase/references/_template.md` as your starting point
+3. Update `skills/supabase/SKILL.md` to reference your new file
+4. Run `npm run build && npm run check`
+
+## Creating Reference Files
+
+```bash
+# Main topic
+skills/supabase/references/{feature}.md
+
+# Sub-topics (optional)
+skills/supabase/references/{feature}/{subtopic}.md
+```
+
+**Examples:**
+
+- `references/auth.md` - Authentication overview
+- `references/auth/nextjs.md` - Auth setup for Next.js
+- `references/storage.md` - Storage overview
+
+## Writing Guidelines
+
+Follow the [Agent Skills Open Standard](https://agentskills.io/) best practices:
+
+1. **Concise is key** - Only include what Claude doesn't already know
+2. **Show, don't tell** - Prefer code examples over explanations
+3. **Progressive disclosure** - Keep SKILL.md lean, put details in reference files
+4. **Concrete examples** - Include runnable code with real values
+5. **Common mistakes first** - Help agents avoid pitfalls
+
+**Good example** (~50 tokens):
+
+```typescript
+// Get user session
+const { data: { session } } = await supabase.auth.getSession();
+```
+
+**Avoid** (~150 tokens):
+
+```markdown
+Sessions are a way to track authenticated users. When a user logs in,
+a session is created. You can get the current session using the
+getSession method which returns a promise...
+```
+
+## Update SKILL.md
+
+Add your reference to the resources table:
+
+```markdown
+| Area         | Resource                | When to Use                    |
+| ------------ | ----------------------- | ------------------------------ |
+| Your Feature | `references/feature.md` | Brief description of use cases |
+```
+
+## Validate
+
+```bash
+npm run validate -- supabase   # Check files
+npm run build -- supabase      # Generate AGENTS.md
+npm run check                  # Format and lint
+```