fix: fail a run that lints nothing, and say so

[MaximeGaudin]

Summary

• An empty directory, a typo'd path, or a plain non-skill file all produced the identical "Nothing to report" success line and exit 0 — a CI job pointed at the wrong directory passed forever, indistinguishable from a clean run.
• A run that discovers no SKILL.md files now fails the way a missing path already does, naming the paths it looked under.

Fixes #118
Also resolves #36 (a non-skill file argument exited 0 with no diagnostic).

Test plan

• New regression test failed before the fix (exit 0 + clean-pass line on an empty dir and a plain file)
• New regression test passes after the fix (exit 3, distinct message on stderr, no clean-pass line)
• Local checks mirror CI and passed before push
• CI green on the PR