feat(sheets): add Connected Sheets reads

[steipete]

Summary

• add read-only Connected Sheets data-source list and describe commands with joined execution status
• add extract discovery, full table descriptions, and bounded value reads keyed by the API's anchor-cell identity
• keep BigQuery access explicitly opt-in through a dedicated Sheets-read-only plus BigQuery-read-only client
• extend metadata JSON, generated command docs and Sheets agent guidance, and add a focused Connected Sheets guide

Proof

• make ci
• go test ./internal/cmd -run 'TestSheetsDataSource(ListAndDescribe|TableListDescribeAndRead)$' -count=1 -v
• built-binary schema and invalid-input checks for the new command tree
• autoreview clean with no accepted/actionable findings

The fixture exercises BigQuery table and query specs, DATA_SOURCE sheet status, extract anchor discovery, and bounded reads. Read-only live Connected Sheets validation was not available because the requested validation account does not currently have the required Sheets and BigQuery grants; no persistent authorization was changed.

Refs #938

[clawsweeper]

🦞👀
ClawSweeper picked this up.

Pull request received. I will update this pull request when review starts.