plan 88: clarify catalog use-case, fix error message

Address Copilot review:

- catalog replacement mismatch: <?catalog?> is a file-index
  generator (needs glob + frontmatter), not an in-document
  heading TOC generator. Add a comparison table, explain that
  <?catalog?> is the right replacement only for index-page uses
  of [[_TOC_]]/[TOC], and note mdsmith has no built-in heading
  TOC generator for the more common in-document case.
- Error message: combine into a single fenced code block (no
  longer split across two inline spans) and name both CommonMark
  and goldmark as failure renderers.

Author: claude

plan/88_toc-directive-migration.md

@@ -5,22 +5,21 @@ status: "🔲"
 summary: >-
   New rule MDS035 that flags renderer-specific
   table-of-contents directives (`[TOC]`,
-  `[[_TOC_]]`, `[[toc]]`, `${toc}`) that render as
-  empty text on CommonMark / goldmark, and
-  suggests mdsmith's `<?catalog?>` directive as
-  the portable replacement.
+  `[[_TOC_]]`, `[[toc]]`, `${toc}`) that render
+  as empty text on CommonMark / goldmark. The
+  diagnostic points authors at mdsmith's
+  `<?catalog?>` directive for the file-index use
+  case; heading-level TOCs have no direct
+  mdsmith equivalent.
 ---
 # TOC directive migration aid
 
 ## Goal
 
-Catch renderer-specific TOC directives. These
-disappear when rendered by CommonMark or
-goldmark. Suggest the [`<?catalog?>`][catalog]
-directive as a portable, regenerable
-replacement.
-
-[catalog]: ../internal/rules/MDS019-catalog/README.md
+Catch renderer-specific TOC directives that do
+not render on CommonMark or goldmark. The
+diagnostic tells authors which use case has a
+mdsmith equivalent and which does not.
 
 ## Context
 
@@ -42,29 +41,49 @@ parse as plain text in a paragraph. The
 directive simply disappears from the rendered
 output, which is a silent failure.
 
+### Heading TOC vs file index
+
+The flagged directives and
+[`<?catalog?>`][catalog] solve different
+problems:
+
+| Directive      | Generates                                 | Input              |
+|----------------|-------------------------------------------|--------------------|
+| `[TOC]` et al. | Table of **headings in the current file** | Current doc        |
+| `<?catalog?>`  | Table of **other files** matching a glob  | Glob + frontmatter |
+
+[catalog]: ../internal/rules/MDS019-catalog/README.md
+
+`<?catalog?>` is the right replacement only
+when a directive is used on an index page to
+list sibling or child documents (e.g. a wiki
+homepage with `[[_TOC_]]` listing all pages in
+the space). For in-document heading TOCs — the
+more common case — mdsmith has no built-in
+generator; the author must either drop the
+directive or maintain a manual list.
+
 ### Why this rule, not MDS034
 
 MDS034 ([plan 86](86_markdown-flavor-validation.md))
 validates syntax support against a declared
 flavor. TOC directives are not "flavor features"
-— they are per-renderer conventions that overlap
-with mdsmith's own
-[`<?catalog?>`][catalog] directive.
-
-mdsmith already solves the portable-TOC problem:
-`<?catalog?>` generates a table in-source,
-regenerated by `mdsmith fix`. A dedicated rule
-for detecting external TOC directives gives
-migrators a direct on-ramp.
+— they are per-renderer conventions with no
+canonical spec and no fix path that applies to
+every call site. A dedicated opt-in rule with a
+diagnostic tailored to the use-case distinction
+above is a better fit than folding them into
+MDS034's fix pipeline.
 
 ### Scope
 
 Flag only the four directives above. Do not try
 to auto-generate a `<?catalog?>` block — the
 right glob and front-matter fields depend on the
 project and are not knowable from the TOC call
-site. The diagnostic is informational: "this
-directive will not render; see catalog rule".
+site. The diagnostic is informational and names
+both the file-index case (points to MDS019) and
+the heading-TOC case (no equivalent).
 
 ## Design
 
@@ -94,19 +113,19 @@ opt-in posture. No settings.
 
 ### Error message
 
-`"[TOC]" does not render on CommonMark; use the`
-`<?catalog?> directive (see MDS019)`
+```text
+[TOC] does not render on CommonMark or goldmark; mdsmith has no heading-TOC generator. For file-index use cases, see <?catalog?> (MDS019).
+```
 
 Severity: `warning`.
 
 ### No auto-fix
 
-The rule is detection-only. Auto-generating a
-`<?catalog?>` block would require knowing the
-project's document glob and front-matter
-schema. The diagnostic links to the catalog
-rule README so the author can craft the
-replacement.
+The rule is detection-only. Whether the right
+replacement is `<?catalog?>`, a manually
+maintained list, or deletion depends on intent
+that is not recoverable from the directive
+alone.
 
 ## Tasks
 
@@ -126,8 +145,9 @@ replacement.
 ## Acceptance Criteria
 
 - [ ] `[TOC]` on its own line produces a
-      diagnostic that names the `<?catalog?>`
-      replacement
+      diagnostic that names both the heading-TOC
+      gap and the `<?catalog?>` file-index
+      alternative
 - [ ] `[[_TOC_]]` on its own line produces the
       same diagnostic
 - [ ] `[[toc]]` on its own line produces the