AGENTS.md: Add retrieval-led reasoning note, simplify doc links #6024

Job	Run time
build-and-check	3m 13s
test-integration-packages/billing	1m 5s
test-integration-packages/internal	1m 5s
test-integration-packages/agent-runtime	51s
test-integration-common	47s
test-integration-cli	51s
test-web	51s
test-packages/agent-runtime	44s
test-common	50s
test-packages/billing	42s
test-integration-sdk	1m 39s
test-sdk	48s
test-.agents	51s
test-packages/internal	50s
test-cli	51s
test-integration-web	47s
	16m 45s

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