Make lint exit non-zero on invalid rules

[jpl1337]

Split out of #241 per review — this is the exit-code fix on its own, no CI changes.

chainsaw lint currently exits 0 even when rules fail to validate, so it cannot be used as a gate. The Command::Lint arm already counts failed — it just never acts on it. This returns an error when that count is non-zero.

$ chainsaw lint --kind chainsaw rules/        # with a malformed rule present
[!] /some/rule.yml: did not find expected key at line N column M, ...
[+] Validated 130 detection rules out of 131
$ echo $?
0   # before this change
1   # after this change

Output on success is unchanged; on failure the existing per-file [!] lines are unchanged and a summary error is added.

Verified locally: cargo build --release + chainsaw lint --kind chainsaw rules/ against current rules/ (131/131) exits 0; reintroducing a malformed rule exits 1.

[alexkornitzer]

Thanks :)