docs(skills): objectstack-query — aggregation rule stops counting the aggregate keys

`rules/aggregation.md` said `fields` is not one of the "six keys"
`engine.aggregate()` accepts and pointed at the calling-convention row,
which now lists eight. A count drifts every time the set moves; the
sentence now says "not one of the keys" and leaves the enumeration to the
row it cites. Net zero lines, no re-wrap.

Claude-Session: https://claude.ai/code/session_01KTZmMfzVzjNvyaLyQ8mHvg
Co-authored-by: Claude <noreply@anthropic.com>

Author: claude

skills/objectstack-query/rules/aggregation.md

@@ -46,7 +46,7 @@ const rows = await engine.aggregate('sale', {
 ```
 
 Never list the grouped fields in `fields`: drivers auto-select every grouped
-field into the result rows, and `fields` is not one of the six keys
+field into the result rows, and `fields` is not one of the keys
 `engine.aggregate()` accepts — it is rejected by name (see the calling
 convention in `SKILL.md`).