feat(sheets): add insert subcommand for rows and columns

[andybergon]

Closes #202

Summary

Adds gog sheets insert <spreadsheetId> <sheet> <rows|cols> <start> [--count N] [--after] to insert empty rows or columns at a specific position in a sheet.

Uses the Sheets API InsertDimensionRequest via batchUpdate, following the same pattern as format and copy-validation-from.

Usage

# Insert 3 empty rows before row 2
gog sheets insert <id> "Sheet1" rows 2 --count 3

# Insert 1 column after column 3
gog sheets insert <id> "Sheet1" cols 3 --after

New flags

Flag	Description	Default
--count	Number of rows/columns to insert	1
--after	Insert after the specified position instead of before	false

Testing

• Unit tests with httptest mock server covering rows before/after, cols before/after, and invalid dimension
• make ci passes (fmt, lint, all tests)

[andybergon]

This PR is ready for review.

Note: I squashed and force-pushed — the initial version only had insert-rows, but since the Sheets API InsertDimensionRequest supports both rows and columns via the Dimension field, I consolidated it into a single insert subcommand that handles both with a rows/cols argument.