Update commands/README.md to reflect completion of all SQL commands

[Vipul1432]

• Updated all command status from 'Coming Soon' to 'Complete'
• Added comprehensive feature descriptions for all newly completed commands:
  • DELETE: Selective deletion, JOINs, safety practices, vs TRUNCATE/DROP comparison
  • GRANT: Object-level, database-level, schema-level permissions, role-based access
  • REVOKE: Permission removal, CASCADE vs RESTRICT, cleanup procedures
  • COMMIT: Transaction management, commit modes, error handling, performance optimization
  • ROLLBACK: Error recovery, rollback scenarios, performance impact, recovery patterns
  • SAVEPOINT: Partial rollback control, complex transaction management, nested operations
• Updated learning path to include all completed commands in logical progression
• All SQL command categories now fully documented and ready for use

Complete SQL Command Reference:
DDL (4/4): CREATE, ALTER, DROP, TRUNCATE
DML (4/4): SELECT, INSERT, UPDATE, DELETE
DCL (2/2): GRANT, REVOKE
TCL (3/3): COMMIT, ROLLBACK, SAVEPOINT
Total: 13 comprehensive SQL command references