v0.9.0 — Advisory dependencies and a parallel-scheduler fix

- New `continue_on_failure: true` task option: a dependent task still runs even if a dependency marked this way fails (GitLab allow_failure-style semantics)
- Fixed the parallel scheduler cancelling every unrelated running/pending task on any failure; only actual dependents of a failure are now skipped
- Resynced llms.txt with the current schema and CLI (install package, flag-based CLI shape, new task fields)

See CHANGELOG.md for the full list of changes.