docs: acknowledge v3 multi-agent setup in README #151

Job	Run time
Preflight (fmt, clippy, check, agents-md)	43s
MSRV (1.95)	19s
Test (ubuntu-latest)	1m 16s
Test (windows-latest)	2m 45s
Determinism	40s
cargo-deny	53s
Docs (build, no deploy)	40s
Test (macos-latest)	1m 42s
Size guard (<25 MiB)	2m 3s
Coverage	1m 34s
	12m 35s

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