Seats keep their former names, so a rename breaks nothing (ADR 0122, phase 2)

Phase 1 made the set data; a rename still broke every reference to the old
name. This adds the stable identity: a seat's canonical name changes and its
old name becomes an alias that resolves to it forever. SeatNamed and the holder
and display matching resolve a name (former or current) to its seat, so a
manifest's claim, a held record, the git-seat lookup and the build machine's
embedded set all go on working unchanged after a rename. seat_alias table
(migration 0035), inventory Aliases/RenameSeat, openInventory loads them, and a
'seat rename <from> <to>' command does the whole thing — one operation, no
rebuild, no re-registration, no freeze. Behaviour-neutral until a seat is
renamed. Validated against postgres.

cmd/mesh-controller/main.go

@@ -116,6 +116,8 @@ func run() error {
 		return pushCommand(ctx, args[1:])
 	case "seats":
 		return seatsCommand(ctx, args[1:])
+	case "seat":
+		return seatCommand(ctx, args[1:])
 	case "status":
 		return statusCommand(ctx, args[1:])
 	case "version":