Measure each fix from the move to fixing to delivered, and say one over its budget (issue 403)

hq ADR 0313: the operator's five minutes start when an agent moves the issue
to fixing, not at the merge. mesh-delivery says on each transition, and in
checks, when the pull request opened, when it merged and the class of what it
moves. mesh-issues keeps the first move to fixing, the fix time with its four
phases once the last fix is delivered, serves fix-times, and says
issue-tracker.fix.<class>.over-budget on the operator's channel, cleared by the
next fix of that class within budget.

modules/mesh-issues/cmd/mesh-issues/claims.go

@@ -145,7 +145,7 @@ func (t *Tracker) askUnheard() error {
 			if since.IsZero() {
 				since = t.now()
 			}
-			if _, err := t.recordFix(pr, d.State, d.Why, d.Merged, since, n); err != nil {
+			if _, err := t.recordFix(pr, d.heard(since), n); err != nil {
 				return err
 			}
 			heard = true