message: feat(server): defaults branch query to "main" branch
diff: extend type Stream {
  commits(limit: Int! = 25, cursor: String): CommitCollection
  - commit(id: String!): Commit
  + commit(id: String): Commit
  branches(limit: Int! = 25, cursor: String): BranchCollection
  - branch(name: String!): Branch
  + branch(name: String = "main"): Branch
  }
  extend type User {
  