Format PullUpdated, and keep the announced base head on a rechecked member

A head checked again through mesh-delivery reached the direct verdict with no
base head, so a head no build judges was answered with no trunk commit and the
merge tool would refuse it (novox/hq ADR 0323).

internal/link/events.go

@@ -221,17 +221,17 @@ type SourceMoved struct {
 // PullUpdated is what the forge announces when an open pull request's head moves — opened, or pushed
 // to (novox/hq to-be 45 §9): what the controller asks the build seat to check before it merges.
 type PullUpdated struct {
-	Owner    string `json:"owner"`
-	Repo     string `json:"repo"`
-	Number   int    `json:"number"`
-	Title    string `json:"title,omitempty"`
-	Base     string `json:"base"`
-	Head     string `json:"head"`
-	Commit   string `json:"head_sha"`
+	Owner  string `json:"owner"`
+	Repo   string `json:"repo"`
+	Number int    `json:"number"`
+	Title  string `json:"title,omitempty"`
+	Base   string `json:"base"`
+	Head   string `json:"head"`
+	Commit string `json:"head_sha"`
 	// BaseSHA is the base branch's head when the forge announced the head (novox/hq ADR 0323): the trunk commit a
 	// verdict the controller gives without the build seat — a change that runs no build — is judged against. Empty
 	// from a forge module from before it.
-	BaseSHA string `json:"base_sha,omitempty"`
+	BaseSHA  string `json:"base_sha,omitempty"`
 	CloneURL string `json:"clone_url"`
 	HTMLURL  string `json:"html_url,omitempty"`
 	// Paths are the files the pull request changes; PathsTruncated says there were more.