AGENTS.md: Add retrieval-led reasoning note, simplify doc links (#489) #6029

Job	Run time
build-and-check	3m 10s
test-integration-packages/internal	1m 10s
test-integration-packages/billing	1m 15s
test-integration-web	39s
test-integration-sdk	1m 29s
test-integration-common	42s
test-integration-packages/agent-runtime	42s
test-sdk	45s
test-integration-cli	45s
test-packages/agent-runtime	48s
test-common	42s
test-.agents	42s
test-packages/billing	45s
test-packages/internal	50s
test-cli	51s
test-web	46s
	16m 1s

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