Accept top-level YAML lists in the cron job loader

[fallintoplace]

Problem

load_jobs() contains a branch for top-level YAML lists, but calls data.get("jobs", []) before reaching it. A file like this therefore raises AttributeError instead of loading the job:

- id: hourly-review
  schedule: 1h
  prompt: Review pending tasks

The loader also assumes every other root and every jobs value has the expected collection type, allowing malformed YAML structures to fail during loading.

Changes

• inspect the YAML root before accessing mapping keys
• accept both a top-level list and the existing jobs: mapping
• reject invalid root and job-collection types with a warning
• add focused regression coverage for accepted and rejected structures

Testing

• exercised load_jobs() with list, invalid-root, and invalid-collection inputs
• compiled the changed Python files
• ran git diff --check