* fix: persist custom provider model and variant deletions
The CLI config.update endpoint deep-merges its payload with existing
global config, so removing a model or reasoning variant from a custom
provider in the UI had no effect on disk. Save payloads now emit null
sentinels for removed IDs, and the Provider schema accepts nullable
record values so stripNulls can delete them during the merge.
Closes#9186
* chore: update kilo-vscode visual regression baselines
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