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/issue.go

@@ -112,6 +112,12 @@ type Issue struct {
 	Triage *Verdict `json:"triage,omitempty"`
 	// Worker is who works on it, while it is diagnosing or fixing (ADR 0284); nil at every other status.
 	Worker *Worker `json:"worker,omitempty"`
+	// FixingFrom is when it was first moved to fixing: where its fix time starts (novox/hq ADR 0313 §1). A
+	// take-over or a later move back to fixing keeps it.
+	FixingFrom *time.Time `json:"fixing-from,omitempty"`
+	// FixTime is its fix time, phase by phase, once the last of its fixes is delivered (ADR 0313 §4): measured,
+	// never set by a verb.
+	FixTime *FixTime `json:"fix-time,omitempty"`
 	// Opening is the id of the call that opened it, so a write whose answer was lost is known as its own.
 	Opening string `json:"opening"`
 	// Imported says that the issue, or its status, was taken from its folder in hq by the import of