Add excel-to-markdown skill and converter script

- Add excel_to_markdown.py script for converting Excel files to Markdown tables
- Create excel-to-markdown skill with SKILL.md documentation
- Support automatic data range detection and flexible output options
- Include features for row/column numbering and partial conversions
- Add skill-creator reference documentation

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

skills/excel-to-markdown/SKILL.md

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+---
+name: excel-to-markdown
+description: Convert Excel (.xlsx, .xls) files to Markdown table format. Use for converting Excel documents to Markdown for display or documentation, extracting and analyzing Excel data in a structured format, processing Excel files with automatic data range detection, generating Markdown tables with optional row/column numbering, or handling partial conversions with specific row/column ranges. Triggered by requests like "convert this Excel to Markdown", "export Excel as MD", "analyze this spreadsheet", or when working with .xlsx/.xls files that need to be viewed or processed as text.
+---
+
+# Excel to Markdown Converter
+
+## Overview
+
+Convert Excel spreadsheets to Markdown table format with automatic data range detection and flexible output options. The converter handles complex multi-sheet workbooks, preserves cell formatting, and can display row/column numbers for easy reference.
+
+## Quick Start
+
+### Basic Conversion
+
+Convert an entire Excel file to Markdown:
+
+```bash
+python3 scripts/excel_to_markdown.py input.xlsx -o output.md
+```
+
+### Convert with Row/Column Numbers
+
+Display Excel row and column identifiers for reference:
+
+```bash
+python3 scripts/excel_to_markdown.py input.xlsx -o output.md --show-rows --show-cols
+```
+
+### Specify Data Range
+
+Convert only a specific portion of the spreadsheet:
+
+```bash
+# Convert first 10 rows and 5 columns
+python3 scripts/excel_to_markdown.py input.xlsx -o output.md -r 10 -c 5
+
+# Start from row 2, column 3, convert 20 rows
+python3 scripts/excel_to_markdown.py input.xlsx -o output.md --start-row 2 --start-col 3 -r 20
+```
+
+## Command Line Options
+
+| Option | Description |
+|--------|-------------|
+| `input_file` | Path to the Excel file (required) |
+| `-o, --output` | Output Markdown file path (optional, prints to console if not specified) |
+| `-r, --max-rows` | Maximum number of rows to convert (auto-detect if not specified) |
+| `-c, --max-cols` | Maximum number of columns to convert (auto-detect if not specified) |
+| `--show-rows` | Display row numbers in the first column |
+| `--show-cols` | Display column letters (A, B, C...) in the first row |
+| `--start-row` | Starting row number (default: 1) |
+| `--start-col` | Starting column number (default: 1) |
+
+## Features
+
+### Automatic Data Range Detection
+
+The converter automatically scans the spreadsheet to identify the actual data range (up to 100 rows and 100 columns by default), ignoring empty trailing rows and columns.
+
+### Cell Content Handling
+
+- Empty cells are rendered as blank
+- Pipe characters (`|`) in cell content are escaped as `\|`
+- Newlines within cells are converted to `<br>` tags
+- All cell values are trimmed of leading/trailing whitespace
+
+### Multi-Sheet Support
+
+By default, the converter processes the active (last selected) sheet in the workbook.
+
+## Common Use Cases
+
+### 1. Data Analysis
+
+Extract Excel data for analysis or processing:
+
+```bash
+python3 scripts/excel_to_markdown.py sales_data.xlsx -o sales_analysis.md
+```
+
+Then analyze the Markdown output or provide insights based on the structured data.
+
+### 2. Documentation
+
+Convert spreadsheets to Markdown for inclusion in documentation:
+
+```bash
+python3 scripts/excel_to_markdown.py specifications.xlsx -o specs.md --show-rows --show-cols
+```
+
+The row/column numbers make it easy to reference specific cells in documentation.
+
+### 3. Quick Preview
+
+Quickly view Excel contents without opening a spreadsheet application:
+
+```bash
+python3 scripts/excel_to_markdown.py report.xlsx
+```
+
+### 4. Partial Data Extraction
+
+Extract specific sections from large spreadsheets:
+
+```bash
+# Get header row and first 10 data rows
+python3 scripts/excel_to_markdown.py large_file.xlsx -o sample.md -r 11
+```
+
+## Script Location
+
+The converter script is located at: `scripts/excel_to_markdown.py`
+
+## Dependencies
+
+The script requires `openpyxl` for Excel file processing:
+
+```bash
+pip install openpyxl
+```