plan 88: backtick <?catalog?> in example error message

Copilot review noted the error message backticked `[TOC]` but
left `<?catalog?>` as bare text, inconsistent with the following
explanation that says directive tokens are backticked. Add
backticks around `<?catalog?>` and update the explanation to
cover both tokens.

Two other comments in the same review batch — claiming the
comparison tables in plans 87 and 88 start each row with `||`
and have an empty leading column — are factually incorrect.
Both tables use a single leading `|`; replies posted on the
threads with the verbatim current content.

Author: claude

plan/88_toc-directive-migration.md

@@ -154,14 +154,16 @@ opt-in posture. No settings.
 ### Error message
 
 ```text
-unsupported TOC directive `[TOC]`; mdsmith has no heading TOC equivalent; use <?catalog?> for file indexes (MDS019)
+unsupported TOC directive `[TOC]`; mdsmith has no heading TOC equivalent; use `<?catalog?>` for file indexes (MDS019)
 ```
 
 The leading word is lowercase and there is no
 trailing punctuation, per the mdsmith error
 message convention in [CLAUDE.md](../CLAUDE.md).
-The literal directive token is backticked so it
-is quoted, not capitalized prose.
+Both the input directive token (`[TOC]`) and
+the suggested replacement directive
+(`<?catalog?>`) are backticked so they read as
+quoted tokens, not prose.
 
 Severity: `warning`.