Add aw.yml default for schedule seed

[Copilot]

Repository manifests could not define the seed used for fuzzy schedule scattering. Compilation now uses an aw.yml default when --schedule-seed is omitted.

• Manifest configuration

  • Add and validate schedule-seed in owner/repo format.
  • Document the field in the manifest reference and specification.

• Compile behavior

  • Apply the manifest value before initializing the compiler.
  • Preserve explicit --schedule-seed precedence.

name: Repo Assist
schedule-seed: github/gh-aw