Add Account-Level Negative Keywords

Purpose: Roll a curated baseline of account-level negative keywords (the ones in Admin → Account Settings → Negative Keywords) across one or many Google Ads accounts, using the confirmed 3-step SharedSet approach.

Type: MUTATION skill. This writes to live accounts. The mutation-safety two-step protocol is not a step in this workflow — it is the workflow: dry-run preview → human reads the plan → approval code generated → human gives the go → execution. Never chain code generation and execution in a single action, and never skip the preview read.