feat(cli): --selector flag scoping lint to subtree #110

Job	Run time
Preflight (fmt, clippy, check, agents-md)	31s
Determinism	42s
Size guard (<25 MiB)	2m 2s
cargo-deny	37s
MSRV (1.95)	19s
Test (macos-latest)	1m 5s
Test (ubuntu-latest)	1m 6s
Docs (build, no deploy)	37s
Test (windows-latest)	2m 58s
Coverage	1m 28s
	11m 25s

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