Add marketplace json (#5)

* feat: add marketplace.json for Claude Code plugin distribution

Add .claude-plugin/marketplace.json to enable distribution of the
postgres-best-practices skill as a Claude Code plugin.

Users can install via:
  /plugin marketplace add supabase/agent-skills
  /plugin install postgres-best-practices@supabase-postgres

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

* feat: add plugin.json manifest for postgres-best-practices skill

Add .claude-plugin/plugin.json inside the skill directory to properly
define this as a Claude Code plugin. This is required by the marketplace
specification - each plugin must have its own plugin.json manifest.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

* chore: update marketplace name and description for broader scope

- Rename marketplace from supabase-postgres to supabase-plugins
- Update description to reflect this as the official Supabase agent skills repo

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

* minor fixes

---------

Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>

.claude-plugin/marketplace.json

@@ -0,0 +1,35 @@
+{
+  "name": "supabase-plugins",
+  "owner": {
+    "name": "Supabase"
+  },
+  "metadata": {
+    "description": "Official Supabase agent skills for Claude Code",
+    "version": "1.0.0"
+  },
+  "plugins": [
+    {
+      "name": "postgres-best-practices",
+      "source": "./skills/postgres-best-practices",
+      "description": "Postgres performance optimization and best practices. Use when writing, reviewing, or optimizing Postgres queries, schema designs, or database configurations.",
+      "version": "1.0.0",
+      "author": {
+        "name": "Supabase"
+      },
+      "repository": "https://github.com/supabase/agent-skills",
+      "license": "MIT",
+      "keywords": [
+        "postgres",
+        "postgresql",
+        "database",
+        "sql",
+        "performance",
+        "optimization",
+        "supabase",
+        "rls",
+        "indexing"
+      ],
+      "category": "database"
+    }
+  ]
+}