v0.6.0

• A --since/--subject run that plans zero mutants now exits before the
  baseline is built or refreshed, so no spec process is spawned. Subject
  discovery and the scope filters run ahead of the baseline (#47).
• Every empty --since/--subject plan prints the normal zero-count summary
  block (all statuses at 0, invalid (discarded): 0) with no
  Mutation score: line, whatever the exit code. The JSON reporter emits its
  normal object with score: null and exit_reason: "empty_plan"; the
  github and stryker-json reporters emit their normal empty output. JSON
  counts now always carries all seven status keys (zero when absent)
  instead of only the statuses seen, so the shape is stable across runs.
  Custom reporters must accept an empty_plan: keyword on summary (#45).
• --allow-empty now judges the --since diff instead of forgiving every
  empty plan. It exits 0 only when the diff touched no candidate source file
  (a changed or untracked .rb inside the scanned paths, outside spec paths,
  not excluded), or when --no-class-level dropped the only class-body code
  the diff touched. A candidate that changed but planned nothing exits 1 and
  the warning names the file(s); a comment-only edit in a source file counts
  as a change, and so does a deletion-only edit (the --since line map
  still ignores pure deletions, but the file now counts as touched). The
  --no-class-level forgiveness requires every candidate file to have
  matched a class-body subject. --subject alone still exits 0
  unconditionally. --changed follows the same rule (#46).