dsh-unified-model: Apply model and thinking intensity to all DSH sessions at once

DeepSeek Harness stores the model and reasoning effort within a single session, and neither the default settings nor the `/model` command can switch all historical sessions simultaneously. This plugin adds a unified control to the composer row to batch-write model, provider, and reasoning effort settings to all writable sessions. It skips already matched items, leaves sub-agent sessions untouched, and validates against `modelCatalog()` before writing. The batch tasks run on the Host and include real-time progress updates and result reports.