Add SQL INSERT output format (-O sql)

[vmvarela]

Description

Add sql to the output format list. Generate SQL INSERT statements from query results.

sql-pipe data.csv -O sql 'SELECT * FROM t WHERE active = 1'
# INSERT INTO t (id, name, active) VALUES (1, 'Alice', 1);
# INSERT INTO t (id, name, active) VALUES (3, 'Carol', 1);

Motivation

Useful for data migration between databases, seeding databases, and creating test fixtures. Currently users must convert query output to INSERT statements manually.

Acceptance Criteria

• Add sql to the OutputFormat enum in src/format.zig
• Generate INSERT INTO <table> (<cols>) VALUES (<vals>); for each row
• --sql-table <name> flag to specify target table name (default: "t")
• String values quoted with single quotes, embedded single quotes escaped
• NULL values rendered as NULL (no quotes)
• Numeric values rendered without quotes
• Boolean values rendered as 0/1 (SQLite convention)
• Integration tests cover the new format
• Help text updated

Implementation Notes

• Similar to other output formatters in format.zig
• Use writeJsonString-style escaping for SQL string values (adapt for single quotes)
• About 40-50 lines of new code