Add some docs for coding agent to understand codebase #5929

Job	Run time
build-and-check	3m 5s
test-integration-packages/billing	1m 5s
test-integration-packages/internal	1m 17s
test-integration-sdk	1m 20s
test-integration-web	45s
test-integration-common	51s
test-integration-cli	42s
test-cli	1m 0s
test-sdk	43s
test-packages/billing	44s
test-packages/agent-runtime	46s
test-.agents	46s
test-packages/internal	53s
test-web	53s
test-common	44s
test-integration-packages/agent-runtime	46s
	16m 20s

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