feat(ibis-design): add argument placement, numbering, and layout guidance

Lessons from IBISing a2030.md's "Good Fences Make Good Agents":
arguments got pinned to the wrong position, a ruling-out comment was
really a con, and several issues were already answered by the design
text. Numbering made terse replies ("2: ...") unambiguous, and one big
fenced block outgrew a screen.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

ibis-design/SKILL.md

@@ -54,6 +54,23 @@ leading symbol on each item says what kind of element it is.
    that's what keeps fallbacks easy to agree on.
 6. **When one issue is really several choices, say so** with a `.`
    comment and split it into sibling `?` sub-issues.
+7. **Attach each argument to the position it bears on.** If it applies
+   equally to the siblings, it belongs higher up, or the issue is
+   several choices (rule 6). A comment that rules a position out is a
+   con, and the position is rejected (`:-`).
+8. **Check the source before raising an issue.** If the design text
+   already answers it, record the issue as resolved and cite that text
+   as the pro.
+9. **Past three levels of nesting, split out a sub-issue.**
+
+## numbering
+
+Number elements so replies can refer to them ("2: …"): issues `1`,
+`2`; positions under them `1a`, `1b`; anything below that `1a-i`,
+`1a-ii`, then `1a-i-a`. The number follows the symbol: `?! 2. …`,
+`:+ 2c. …`. Renumbering breaks references made in chat or other docs,
+so append rather than renumber once people are citing numbers. Show
+dependencies between issues with "(see 5)".
 
 ## writing it in markdown
 
@@ -62,6 +79,12 @@ auto-bulleting. Put each symbol after the list marker, escaping where
 needed (`- \- con`, `- \+ pro`), or use a fenced block with indentation
 as in the example below.
 
+Past a screenful, give each top-level issue its own heading
+(`### ? 3. What is the fence?`) with its elements in a fenced block
+below. Headings show up in the editor outline and give each issue a
+link target. Links don't render inside a fence, so put sources after
+the block and refer to them from the line they support.
+
 ## example
 
 ```