feat: support source-level skip-if-command

[magrathean-uk]

Summary

Adds the profile-only skip-if-command protocol: exact skip omits a source, empty output continues, and other output or command failure stops with an error.

Validation

• 4 directive/failure unit tests
• Configuration reference and full-profile example
• cargo clippy --locked --all-targets --features release -- -D warnings
• cargo fmt --all -- --check
• git diff --check
• typos

Fixes #1524.