Add worksheet selection to excel-to-markdown skill

Add --sheet parameter to specify worksheets by name or index (1-based). Change default behavior to use first worksheet instead of active sheet, with proper error messages for invalid worksheet references.

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

skills/excel-to-markdown/SKILL.md

@@ -19,6 +19,18 @@ Convert an entire Excel file to Markdown:
 python3 scripts/excel_to_markdown.py input.xlsx -o output.md
 ```
 
+### Specify Worksheet
+
+Convert a specific worksheet by name or index:
+
+```bash
+# By worksheet name
+python3 scripts/excel_to_markdown.py input.xlsx -o output.md --sheet "Sheet2"
+
+# By worksheet index (1-based)
+python3 scripts/excel_to_markdown.py input.xlsx -o output.md --sheet 2
+```
+
 ### Convert with Row/Column Numbers
 
 Display Excel row and column identifiers for reference:
@@ -51,6 +63,7 @@ python3 scripts/excel_to_markdown.py input.xlsx -o output.md --start-row 2 --sta
 | `--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) |
+| `--sheet` | Worksheet name or index (default: first worksheet) |
 
 ## Features
 
@@ -67,7 +80,12 @@ The converter automatically scans the spreadsheet to identify the actual data ra
 
 ### Multi-Sheet Support
 
-By default, the converter processes the active (last selected) sheet in the workbook.
+By default, the converter processes the first worksheet in the workbook. You can specify a different worksheet using the `--sheet` parameter:
+
+- **By name**: `--sheet "Sheet2"` or `--sheet "Data"`
+- **By index**: `--sheet 2` (1-based indexing)
+
+If the specified worksheet is not found, an error will display the list of available worksheets.
 
 ## Common Use Cases