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_test.go

@@ -415,3 +415,45 @@ func TestNoVerbSetsTheOperatorsKeyOrRevealsASecret(t *testing.T) {
 		t.Fatalf("behind %v: %v", behind, err)
 	}
 }
+
+// A value with a quote in it can be said on a `command` line, as in a shell, and a line whose quote
+// is left open is refused naming where it opened and how a quote is written (novox/hq issue 294: an
+// apostrophe inside a single-quoted JSON value cut the line, and the refusal named no cause).
+func TestAQuotedValueCanHoldAQuote(t *testing.T) {
+	for _, c := range []struct{ line, want string }{
+		{`settings set claude-code '{"role":"the operator'\''s laptop"}'`, `{"role":"the operator's laptop"}`},
+		{`settings set claude-code "{\"role\":\"the operator's laptop\"}"`, `{"role":"the operator's laptop"}`},
+		{"x \"a \\\\ b\nc\"", "a \\ b\nc"},
+		{`x 'a\b'`, `a\b`},
+	} {
+		argv, err := splitCommandLine(c.line)
+		if err != nil || argv[len(argv)-1] != c.want {
+			t.Errorf("%s: read back %q %v, want %q", c.line, argv, err, c.want)
+		}
+	}
+
+	_, err := splitCommandLine(`settings set claude-code '{"role":"the operator's laptop"}' --node g14`)
+	if err == nil {
+		t.Fatal("an apostrophe that leaves a quote open was accepted")
+	}
+	for _, want := range []string{"character 57", `"}' --node g14`, `'\''`, `\"`} {
+		if !strings.Contains(err.Error(), want) {
+			t.Errorf("the refusal does not say %q: %v", want, err)
+		}
+	}
+}
+
+// Every value reads back as it was when written the way the refusal says: single-quoted with '\”
+// for each quote, or double-quoted with \ before each " and \ — newlines and backslashes included.
+func TestAQuotedValueRoundTrips(t *testing.T) {
+	for _, v := range []string{`plain`, `it's`, `say "hi"`, `back\slash\`, "two\nlines", `'"\'\"`, `''`, ``} {
+		single := "x '" + strings.ReplaceAll(v, "'", `'\''`) + "'"
+		double := `x "` + strings.NewReplacer(`\`, `\\`, `"`, `\"`).Replace(v) + `"`
+		for _, line := range []string{single, double} {
+			argv, err := splitCommandLine(line)
+			if err != nil || len(argv) != 2 || argv[1] != v {
+				t.Errorf("%s: read back %q %v, want %q", line, argv, err, v)
+			}
+		}
+	}
+}