ai(rules[AGENTS]): allow optional blank line above 'what:' in commit body

why: the commit-message format documented as
"why: ...
what: ..." reads tightly when the why-block is one line, but visually
collapses when why runs to multiple paragraphs and the eye loses the
boundary between explanation and change-list. A blank line between
the two sections fixes that without changing the structure or any
tooling that parses the keys.

what:
- AGENTS.md: insert a blank line in the format template between the
  why: and what: sections, and add a one-paragraph note that the
  blank line is optional and useful for multi-line why-blocks
- Update the two worked examples (config[merge] and the heredoc
  Component[method] template) to show the blank line for consistency

Author: tony

AGENTS.md

@@ -603,11 +603,16 @@ Format commit messages as:
 Scope(type[detail]): concise description
 
 why: Explanation of necessity or impact.
+
 what:
 - Specific technical changes made
 - Focused on a single topic
 ```
 
+The blank line between the `why:` block and the `what:` block is
+optional — useful when the `why:` body runs to multiple lines and the
+two sections benefit from visual separation.
+
 Common commit types:
 - **feat**: New features or enhancements
 - **fix**: Bug fixes
@@ -627,6 +632,7 @@ Example:
 config(feat[merge]): Add deep-merge support for theme options
 
 why: Enable per-project theme overrides without replacing entire dict
+
 what:
 - Add deep_merge() helper for nested dict merging
 - Update merge_sphinx_config() to deep-merge theme_options
@@ -638,6 +644,7 @@ git commit -m "$(cat <<'EOF'
 feat(Component[method]) add feature description
 
 why: Explanation of the change.
+
 what:
 - First change
 - Second change