Add Markdown table output format (-O markdown)

[vmvarela]

Description

Add markdown (alias md) to the output format list. Render SQL query results as a GitHub-Flavored Markdown table.

sql-pipe data.csv -O markdown 'SELECT region, SUM(amount) FROM t GROUP BY region'
# | region | SUM(amount) |
# |--------|-------------|
# | East   | 250         |
# | West   | 250         |

Motivation

Markdown tables are the standard format for embedding data in documentation, READMEs, GitHub issues, PRs, Slack/Teams messages, and Jupyter notebooks. Currently users must use --table + manual cleanup or pipe through another tool. This is the most-requested output format for CLI tools that produce tabular data.

Acceptance Criteria

• Add markdown to the OutputFormat enum in src/format.zig
• Implement markdown table writer with proper pipe-delimited rows and header separator
• Left-align text columns, right-align numeric columns (reuse table.zig numeric detection)
• NULL values render as empty cells (not the string "NULL")
• -O md works as a shorthand alias
• Compatible with --header (always include header for markdown)
• Integration tests cover the new format
• Help text updated

Implementation Notes

• The two-pass approach from table.zig (compute widths, then print) can be reused almost directly
• About 40-50 lines of new code
• Markdown doesn't strictly need visual alignment (pipes are the structure), but aligned output looks better