feat(cli): Add markdown table upsert command

[arcaputo3]

Summary

Add support for importing markdown tables into Excel, similar to the existing CSV import.

Motivation

From agent feedback testing (v0.5.0 review):

Building JSON for every cell is the slowest part of model construction. A CSV or markdown upsert would cut work in half.

While CSV import was added in v0.5.x, markdown tables are more natural for LLM agents to generate and reason about.

Proposed Usage

# From file
xl -f file.xlsx -s Sheet1 -o out.xlsx import-md table.md --start A1

# From stdin
echo "| A | B |
|---|---|
| 1 | 2 |" | xl -f file.xlsx -s Sheet1 -o out.xlsx import-md - --start A1

Implementation Notes

• Parse GFM (GitHub Flavored Markdown) table syntax
• Handle alignment markers (:---, :---:, ---:)
• Auto-detect numeric values vs text (like CSV import)
• Consider --header flag to treat first row as headers