Add optional Model string to pkg/agent/config.CloudAssistantConfig (#… #2484

Job	Run time
Lint	3m 56s
Build and test with Go 1.24 on ubuntu-latest	2m 26s
Build and test with Go 1.24 on windows-latest	3m 9s
Test parser against vast amount of Markdowns	1m 37s
Test in Docker	2m 54s
	14m 2s

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