feat(sheets): expose spreadsheets.batchUpdate for formatting and structural edits #450

What happened

The sheets service exposes only read tools (getText, getRange, getMetadata). There is no tool to apply any formatting or structural edit to a spreadsheet — bold, colors, number formats, borders, column widths, text wrapping, frozen rows, conditional formatting, merges, or sheet properties.

Grounded in the source: the sheets.write feature group in feature-config.ts is empty (tools: []) and defaultEnabled: false, so no write tool is registered. The underlying Google spreadsheets.batchUpdate endpoint is only called internally (to build addSheet/deleteSheet requests); it is not surfaced as a tool. Any attempt to apply formatting via batchUpdate therefore has no MCP tool to route to.

This is distinct from #291 (Sheets write/create support), which covered value/structure write operations. Formatting is a separate, still-missing primitive.

What I expected

A tool that exposes spreadsheets.batchUpdate so formatting and structural edits can be applied — matching the precedent of slides.updateShapeProperties, which takes a JSON request string plus a field mask.

Proposed scope

• Add SheetsService.batchUpdate (accepts a requests JSON string, validates Request[], calls spreadsheets.batchUpdate).
• Register it as a write tool in index.ts and add it to the sheets.write group in feature-config.ts, flipping the group defaultEnabled: true consistent with the other write groups.
• Coupled test updates for the two tests that encoded the old sheets.write = OFF default, plus new batchUpdate coverage.

A fix is in flight.