fix: address docs checker review findings

Check every link reference definition, decode destinations per CommonMark,
report correct lines for empty headings, autolinks and nested definitions,
reject symlinked record directories and wrong-case path components, and
refuse titles that cannot pass. Only top-level headings count (spec section 6).

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

docs/docs-format.md

@@ -116,7 +116,7 @@ Plan `status`:
 
 ## 6. Body
 
-The body is Markdown (CommonMark) after the frontmatter. Headings inside fenced code blocks or HTML comments do not count. Required section headings are matched case-sensitively and exactly, at level 2 (`##`). A section runs until the next heading of level 1 or 2, so its subsections belong to it. A required section MUST appear exactly once and contain non-empty prose: at least one paragraph (including paragraphs inside blockquotes), list item or table cell of text. HTML comments and code (fenced, indented or inline) do not count as prose. Extra sections are allowed anywhere.
+The body is Markdown (CommonMark) after the frontmatter. Headings inside fenced code blocks or HTML comments do not count. Only headings at the top level of the document count. Required section headings are matched case-sensitively and exactly, at level 2 (`##`). A section runs until the next heading of level 1 or 2, so its subsections belong to it. A required section MUST appear exactly once and contain non-empty prose: at least one paragraph (including paragraphs inside blockquotes), list item or table cell of text. HTML comments and code (fenced, indented or inline) do not count as prose. Extra sections are allowed anywhere.
 
 Heading text is the raw source after the `#` markers, trimmed, with any closing `#` run removed; it is compared byte for byte, with no inline-markup rendering. Setext headings are read as the equivalent ATX heading.