docs(skills): admit hand-authored reference files under references/ (#14786)

The catalog anatomy in `skills/README.md` reserved `references/` for the
generated `_index.md` alone and assigned every hand-authored per-topic file to
`rules/`. Three published skills already keep hand-authored anchors under
`references/` (`objectstack-data/references/data-hooks.md`,
`objectstack-platform/references/plugin-hooks.md`), each linked directly from
its own `SKILL.md` and cited by path from sibling packages, so the anatomy
described a layout the catalog does not have.

Amend the anatomy rather than move the files: the reader reaches the same text
either way, and a move would be a multi-package rename with binding checks and
ceiling-row renames for zero reader value. `_index.md` keeps its "generated —
do not hand-edit" line and its `gen:skill-refs` pointer, byte-identical;
`rules/` keeps its per-topic role. The requirement travels with the permission:
a hand-authored reference file must be linked from `SKILL.md`.


Claude-Session: https://claude.ai/code/session_01LraLgQVGq8egUwfYZpbYt1

Co-authored-by: Claude <noreply@anthropic.com>

Author: os-litant

skills/README.md

@@ -52,7 +52,7 @@ apps too).
 ```
 skills/<skill-name>/
 ├── SKILL.md              # frontmatter + prose guide
-├── references/
+├── references/           # may also hold hand-authored reference files linked from SKILL.md
 │   └── _index.md         # generated pointers into @objectstack/spec sources
 │                         # (pnpm --filter @objectstack/spec gen:skill-refs — do not hand-edit)
 ├── rules/                # (optional) detailed per-topic rule files linked from SKILL.md