feat(sheets): add chart subcommand (create, list, get, update, delete)

[andybergon]

Summary

• Adds sheets chart with list/get/create/update/delete subcommands
• Charts managed via --spec-json passthrough of EmbeddedChartSpec JSON, keeping scope small while covering all chart types
• create supports --sheet, --anchor, --width, --height for positioning
• delete requires --force or interactive confirmation

Examples

# List all charts
gog sheets chart list <spreadsheetId>

# Get full chart definition (spec + position)
gog sheets chart get <spreadsheetId> <chartId> --json

# Backup a chart, delete it, and recreate it
gog sheets chart get <spreadsheetId> 563157883 --json > chart.json
gog sheets chart delete <spreadsheetId> 563157883 --force
gog sheets chart create <spreadsheetId> --spec-json @chart.json

# Create with explicit positioning
gog sheets chart create <spreadsheetId> --spec-json @chart.json --sheet Sheet1 --anchor E10 --width 800 --height 400

# Update a chart's spec (e.g. change title and type)
gog sheets chart update <spreadsheetId> <chartId> --spec-json '{"title":"Revenue Q1","basicChart":{"chartType":"PIE"}}'

Test plan

• 15 unit tests covering all commands, JSON/text output, anchor positioning, validation, dry-run, and destructive confirmation
• Manually tested against a real spreadsheet: list, get, delete, create, update all working end-to-end

Closes #432