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-knowledge/module.json

@@ -87,6 +87,7 @@
     "seat:operator-channel.notify",
     "seat:issue-tracker.list",
     "seat:issue-tracker.get",
+    "seat:issue-tracker.milestones",
     "seat:mesh-delivery.deliveries",
     "seat:mesh-delivery.show",
     "seat:mesh-delivery.groups"