Detect Ghostty and default to dark mode #4018

Job	Run time
build-and-check	2m 32s
test-cli	32s
test-web	32s
test-backend	40s
test-.agents	35s
test-common	31s
test-integration-backend	34s
test-integration-.agents	38s
test-integration-common	39s
test-packages/agent-runtime	38s
test-integration-npm-app	31s
test-npm-app	37s
test-integration-cli	34s
test-integration-web	38s
test-integration-packages/agent-runtime	33s
	10m 44s

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