AGENTS.md: Add retrieval-led reasoning note, simplify doc links #6027

Job	Run time
build-and-check	3m 20s
test-integration-packages/billing	1m 5s
test-integration-packages/agent-runtime	41s
test-integration-cli	43s
test-integration-web	45s
test-integration-sdk	1m 28s
test-packages/agent-runtime	47s
test-cli	53s
test-integration-packages/internal	1m 3s
test-web	50s
test-packages/billing	42s
test-sdk	51s
test-integration-common	49s
test-common	43s
test-packages/internal	51s
test-.agents	45s
	16m 16s

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