mesh-issues and mesh-knowledge: milestones are records of the issue tracker, and progress reads them (ADR 0493 §1 to §4, issue 1781)

mesh-issues keeps each design's milestone in the state milestones (history 5), says milestone-set and
milestone-moved, and serves milestones, milestone-set and milestone-move. An issue's milestone is its own link
of kind milestone, so at most one milestone per issue is checked inside the issue's own write; kind commit
links a fixing commit. The triage links no milestone, pull request or commit. import with milestones=N seeds
a design's table once. mesh-knowledge's progress reads issue-tracker.milestones and its table parse is gone.

Squashed from four commits so the branch history does not carry a compiled bundle that was committed by
mistake (6 MB); the path is now in .gitignore.

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

@@ -28,7 +28,7 @@ type memory struct {
 }
 
 func newMemory() *memory {
-	return &memory{data: map[string]map[string]Entry{IssuesState: {}, CountersState: {}, GroomingState: {}, ReservationsState: {}}, fail: map[string]error{},
+	return &memory{data: map[string]map[string]Entry{IssuesState: {}, CountersState: {}, GroomingState: {}, ReservationsState: {}, MilestonesState: {}}, fail: map[string]error{},
 		hist: map[string]map[string][]Entry{}}
 }