Add comprehensive skills and documentation for various technologies

.claude/skills/databases/mongodb/SKILL.md

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+# MongoDB
+
+## Description
+
+MongoDB patterns including document design, queries, and aggregation.
+
+## When to Use
+
+- MongoDB database operations
+- Document-based data modeling
+- Aggregation pipelines
+
+---
+
+## Core Patterns
+
+### Document Operations
+
+```javascript
+// Insert
+db.users.insertOne({
+  email: 'user@example.com',
+  name: 'John',
+  createdAt: new Date()
+});
+
+// Find
+db.users.find({ active: true }).sort({ createdAt: -1 }).limit(20);
+
+// Update
+db.users.updateOne(
+  { _id: ObjectId('...') },
+  { $set: { name: 'Jane' } }
+);
+```
+
+### Aggregation
+
+```javascript
+db.orders.aggregate([
+  { $match: { status: 'completed' } },
+  { $group: {
+    _id: '$userId',
+    totalSpent: { $sum: '$amount' },
+    orderCount: { $count: {} }
+  }},
+  { $sort: { totalSpent: -1 } }
+]);
+```
+
+### Indexes
+
+```javascript
+// Single field
+db.users.createIndex({ email: 1 }, { unique: true });
+
+// Compound
+db.posts.createIndex({ userId: 1, createdAt: -1 });
+```
+
+## Best Practices
+
+1. Embed frequently accessed data
+2. Use references for large/independent data
+3. Create indexes for query patterns
+4. Use aggregation for complex queries
+5. Avoid unbounded arrays
+
+## Common Pitfalls
+
+- **Unbounded arrays**: Limit array size
+- **Missing indexes**: Analyze query patterns
+- **Over-embedding**: Consider data access patterns

.claude/skills/databases/postgresql/SKILL.md

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+# PostgreSQL
+
+## Description
+
+PostgreSQL database patterns including queries, indexing, and optimization.
+
+## When to Use
+
+- PostgreSQL database operations
+- SQL query optimization
+- Schema design
+
+---
+
+## Core Patterns
+
+### Basic Queries
+
+```sql
+-- Select with filtering
+SELECT id, name, email
+FROM users
+WHERE active = true
+ORDER BY created_at DESC
+LIMIT 20 OFFSET 0;
+
+-- Join
+SELECT u.*, COUNT(p.id) as post_count
+FROM users u
+LEFT JOIN posts p ON p.user_id = u.id
+GROUP BY u.id;
+```
+
+### Indexes
+
+```sql
+-- Single column index
+CREATE INDEX idx_users_email ON users(email);
+
+-- Composite index
+CREATE INDEX idx_posts_user_date ON posts(user_id, created_at DESC);
+
+-- Partial index
+CREATE INDEX idx_active_users ON users(email) WHERE active = true;
+```
+
+### Migrations
+
+```sql
+-- Add column with default
+ALTER TABLE users ADD COLUMN role VARCHAR(50) DEFAULT 'user';
+
+-- Add constraint
+ALTER TABLE users ADD CONSTRAINT unique_email UNIQUE (email);
+```
+
+## Best Practices
+
+1. Use indexes for filtered/sorted columns
+2. Use EXPLAIN ANALYZE for slow queries
+3. Avoid SELECT * in production
+4. Use transactions for multiple operations
+5. Use connection pooling
+
+## Common Pitfalls
+
+- **N+1 queries**: Use JOINs or batch loading
+- **Missing indexes**: Add indexes for WHERE/ORDER BY
+- **Large transactions**: Keep transactions short