Add opus agent to slash commands. Change to /agent: format #5541

Job	Run time
build-and-check	4m 0s
test-integration-packages/billing	1m 9s
test-integration-packages/internal	1m 7s
test-integration-web	42s
test-integration-packages/agent-runtime	38s
test-integration-sdk	38s
test-integration-cli	41s
test-integration-common	46s
test-packages/agent-runtime	59s
test-common	38s
test-.agents	39s
test-sdk	48s
test-web	45s
test-packages/internal	48s
test-cli	1m 11s
test-packages/billing	40s
	16m 9s

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