Description

Currently sql-pipe only outputs CSV. Adding JSON output (--json) would make the tool much more useful in pipelines with jq, APIs, and modern CLI workflows.

Acceptance Criteria

• --json flag outputs results as a JSON array of objects
• Each row is an object with column names as keys
• Numeric values are JSON numbers (not strings)
• NULL values are represented as JSON null
• Empty result set outputs []
• --json is mutually exclusive with --delimiter / --tsv / --header
• Man page and README updated

Notes

• Example: echo "name,age\nAlice,30" | sql-pipe "SELECT * FROM stdin" --json → [{"name":"Alice","age":30}]
• Zig's std.json can handle serialization
• Complements the existing CSV/TSV output modes
• Dependency note: This issue is a subset of Multi-format input/output support (JSON, TSV, Parquet, etc.) #68 (multi-format I/O). Implement --json here first as a standalone flag; Multi-format input/output support (JSON, TSV, Parquet, etc.) #68 will later generalize it into --output-format json alongside other formats. Avoid designing a full format plugin system here — keep it simple.