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/tracker.go

@@ -7,6 +7,7 @@ import (
 	"errors"
 	"fmt"
 	"math/big"
+	"slices"
 	"sort"
 	"strings"
 	"sync"
@@ -650,6 +651,22 @@ func (t *Tracker) link(by string, a LinkAsk) (*Linked, error) {
 			}
 		}
 	}
+	if by == GroomBy && slices.Contains(madeByPeople, kind) {
+		return nil, fmt.Errorf("the triage links no %s: that link is made by the one who did the work (ADR 0493 §4)", kind)
+	}
+	if kind == LinkMilestone && !a.Remove {
+		d, id, _ := parseMilestoneRef(target)
+		m, _, err := t.readMilestone(d, id)
+		if err != nil {
+			return nil, err
+		}
+		if m == nil {
+			return nil, fmt.Errorf("there is no milestone %s: it is made from its design's table (milestone-set, or import with milestones)", target)
+		}
+		if m.Status != MilestoneOpen {
+			return nil, fmt.Errorf("milestone %s is %s: a fix found on the way joins its design's open milestone, or a milestone of its own (ADR 0493 §3)", target, m.Status)
+		}
+	}
 	at := t.now()
 	is, err := t.change(a.Number, Act{Op: EventLinked, By: by}, func(is *Issue) error {
 		for i, l := range is.Links {
@@ -665,6 +682,12 @@ func (t *Tracker) link(by string, a LinkAsk) (*Linked, error) {
 		if a.Remove {
 			return fmt.Errorf("issue %d has no %s link to %s to remove", is.Number, kind, target)
 		}
+		if kind == LinkMilestone {
+			if in := milestoneOf(is); in != "" {
+				return fmt.Errorf("issue %d is in milestone %s already, and an issue belongs to at most one (ADR 0493 §2): "+
+					"remove that link first to move it", is.Number, in)
+			}
+		}
 		if len(is.Links) >= MostLinks {
 			return fmt.Errorf("issue %d has %d links, the most one issue keeps", is.Number, MostLinks)
 		}