Every code repo's main head (for followers; no Artifacts read)
Main as Pando last moved it: by the first push through the door, or by
a merge. Answered from the monorepo object's records. A repo made
before Pando kept heads is read once from Artifacts and kept. head is
null for an empty repo. The meta layer's repo is not listed.
A follower (pando worktree follow) calls this on connect, then opens
GET /v1/monorepos/{owner}/{monorepo}/events?topics=main&since=<seq> (WebSocket) to
hear each move, and calls it again every few minutes as a safety net.
GET
https://app.gitpando.com/v1/monorepos/{owner}/{monorepo}/heads