Say where a command line's quote is left open and how a quote is written (hq issue 294)

An apostrophe inside a single-quoted value ends that quote, and the line then failed as a bare
'unclosed quote' with no position and no way out named. The splitter's rules stay the shell's; the
refusal now names the character, shows the line from there and gives the two ways to write a quote.

cmd/mesh-controller/seatverbs.go

@@ -1366,7 +1366,7 @@ func splitCommandLine(line string) ([]string, error) {
 	var words []string
 	var cur strings.Builder
 	inWord := false
-	quote := rune(0)
+	quote, opened := rune(0), 0
 	runes := []rune(line)
 	for i := 0; i < len(runes); i++ {
 		r := runes[i]
@@ -1387,7 +1387,7 @@ func splitCommandLine(line string) ([]string, error) {
 				cur.WriteRune(r)
 			}
 		case r == '\'' || r == '"':
-			quote = r
+			quote, opened = r, i
 			inWord = true
 		case r == '\\' && i+1 < len(runes):
 			i++
@@ -1405,7 +1405,7 @@ func splitCommandLine(line string) ([]string, error) {
 		}
 	}
 	if quote != 0 {
-		return nil, fmt.Errorf("command has an unclosed %c quote", quote)
+		return nil, unclosedQuote(runes, quote, opened)
 	}
 	if inWord {
 		words = append(words, cur.String())
@@ -1413,6 +1413,22 @@ func splitCommandLine(line string) ([]string, error) {
 	return words, nil
 }
 
+// unclosedQuote is the refusal of a line whose quote is never closed. Most often an apostrophe inside
+// a single-quoted value ended that quote early and a later quote was left open, so the refusal says
+// where the open quote is, shows the line from there, and says how a quote is written inside a quoted
+// value — the shell's own two ways, which this splitter already reads (novox/hq issue 294).
+func unclosedQuote(runes []rune, quote rune, opened int) error {
+	const shown = 40
+	from := runes[opened:]
+	excerpt := string(from)
+	if len(from) > shown {
+		excerpt = string(from[:shown]) + "…"
+	}
+	return fmt.Errorf("command has an unclosed %c quote, opened at character %d: %s — an apostrophe "+
+		"inside a single-quoted value ends it; write a ' inside single quotes as '\\'' (it'\\''s), or use "+
+		"double quotes and write \\\" for a \" and \\\\ for a \\ inside them", quote, opened+1, excerpt)
+}
+
 // seatAnnouncement is what the controller says it serves on the bus (novox/hq ADR 0197): the
 // mesh-controller seat, one endpoint per verb it answers, each with the seat's own description and
 // argument schema — the same facts `tools` answers from the records, as NATS's services format.