chore: remove external runtime repo dependency from supported Dispatch workflows

[joryirving]

Parent: #392

Make Dispatch itself the source of truth for the supported agent workflow contract.

Goal:

• external repos may contain examples or experiments
• required runtime behavior lives in Dispatch docs and APIs
• users should not need a second repo to understand the worker or groomer loop

Acceptance:

• audit docs for references to required external prompt/script repos
• move required contract docs into Dispatch
• mark external workflow repos as optional examples only
• keep harness-specific examples small and non-authoritative