fix(util): rune-safe truncation across formatters

firstLineOf (query) and truncateString (edit) sliced strings at a raw byte
offset, splitting a multibyte UTF-8 character into invalid bytes in
LLM-facing output (ast_query compact format and edit not-found errors are
hot paths). Extract the rune-boundary backoff already used by search's
truncateLine into util.RuneBoundary and route all three sites through it.

Adds table-driven tests for the primitive and a multibyte case for firstLineOf.

internal/query/format_test.go

@@ -186,6 +186,9 @@ func TestFirstLineOf(t *testing.T) {
 		{"line1\nline2", 20, "line1"},
 		{"\n\nfoo", 20, "foo"},
 		{"abcdefghij", 5, "abcd…"},
+		// Multibyte: "ααα" is 6 bytes (each α = 2). maxLen-1 = 3 lands inside the
+		// 2nd α; must back off to a rune boundary, never split into invalid UTF-8.
+		{"ααα", 4, "α…"},
 	}
 	for _, c := range cases {
 		got := firstLineOf(c.input, c.maxLen)

internal/query/query.go

@@ -583,7 +583,7 @@ func firstLineOf(s string, maxLen int) string {
 			continue
 		}
 		if len(line) > maxLen {
-			return line[:maxLen-1] + "…"
+			return line[:util.RuneBoundary(line, maxLen-1)] + "…"
 		}
 		return line
 	}