docs(skill): add rules-writing conventions

Add rules.md covering document structure, theme/players/time header, icon-first mentions, veteran-first prose, and a closing timestamp. Link it from SKILL.md.

.agents/skills/tttk-authoring/SKILL.md

@@ -84,7 +84,8 @@ marker, and fence attribute keys may contain hyphens (e.g. `data-foo=bar`):
   template layers or card backs.
 - To print a rules document, use `:md-print[./rules.md]` (or the `yaml role=tag`
   form). It paginates into A4 landscape sheets holding two A5 pages, each with
-  `columns` columns (default 2). See `reference.md` for the attributes.
+  `columns` columns (default 2). See `reference.md` for the attributes and
+  `rules.md` for how to write the rules text itself.
 - Keep CSV files clean — header row plus data. Put deck config and templates in
   the YAML block's `data-config`, not in CSV front matter (see `csv.md`).
 - `{{prop}}` in a CSV `body` interpolates another column (or front-matter key)
@@ -138,6 +139,9 @@ directory. The full code catalog lives in `reference.md`.
 - `csv.md` — CSV authoring: columns, `{{prop}}` interpolation, quoting, and the
   discouraged front-matter form.
 - `layout.md` — column layout markers.
+- `rules.md` — conventions for writing rules documents: the five-part
+  structure, the theme/players/time header, icon-first mentions, veteran-first
+  prose with details boxed in alerts, and a timestamp.
 - `markdown.md` — GitHub alerts and mermaid diagrams.
 - `examples/adventure.md` and `examples/encounters.csv` — a complete minimal
   document and its data file.