feat: add postgresql-table-design skill to database-design plugin (#113)

Add comprehensive PostgreSQL schema design skill covering:
- Core rules and best practices
- PostgreSQL-specific gotchas and conventions
- Data types with detailed guidance (IDs, strings, time, JSONB, arrays, etc.)
- Indexing strategies (B-tree, GIN, GiST, BRIN, partial, covering)
- Constraints (PK, FK, UNIQUE, CHECK, EXCLUDE)
- Partitioning patterns
- Performance optimization for update-heavy and insert-heavy workloads
- Safe schema evolution
- Extension recommendations

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

.claude-plugin/marketplace.json

@@ -1055,6 +1055,9 @@
       "agents": [
         "./agents/database-architect.md",
         "./agents/sql-pro.md"
+      ],
+      "skills": [
+        "./skills/postgresql"
       ]
     },
     {
@@ -2018,4 +2021,4 @@
       ]
     }
   ]
-}
+}