Allow setting per-marketplace autoUpdate from the CLI

Preflight Checklist

• [x] I have searched existing feature requests and found no duplicate.
• [x] This is a single feature request.

Problem Statement

Per-marketplace autoUpdate has no non-interactive write path. The /plugin toggle needs a human at a terminal, and claude plugin marketplace add offers only --scope and --sparse. Managed settings cover an organization but not an individual, and there is no claude config command. A provisioning script therefore cannot enable auto-update for the marketplace it just added.

Proposed Solution

Add a write path to the CLI, either a flag on the existing command:

claude plugin marketplace add acme/plugins --auto-update

or a dedicated setter, which also allows changing an existing marketplace:

claude plugin marketplace set-auto-update acme/plugins true

Alternative Solutions

Hand-editing extraKnownMarketplaces in settings.json is the only route today, and it is unreliable: marketplace add rewrites that entry and drops the key (#91271), so a script must re-assert it after every add. Doing that from shell means a jq or python dependency purely to set one boolean.

#85844 asks to surface this field in the /plugin UI and in list --json. That covers reading and interactive editing; this request covers setting it from a script.

Priority

Low. A workaround exists, but it requires editing Claude Code's own settings file from outside Claude Code.