feat: add Markdown table output format (-O markdown)

[vmvarela]

Summary

Adds markdown (alias md) as a new output format for rendering SQL query results as GitHub-Flavored Markdown tables.

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

Changes

• src/format.zig — added markdown to OutputFormat enum, md alias in parse()
• src/markdown.zig — new two-pass writer (width computation + numeric detection, then aligned output)
• src/main.zig — markdown dispatch in execQuery()
• src/args.zig — help text updated
• build.zig — 6 integration tests (basic output, md alias, numeric alignment, NULL handling, aggregation, empty result)
• docs/sql-pipe.1.scd — man page updated with markdown option + example
• README.md — flags table updated

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 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

Closes #167