fix: restructure plugin so Claude Code loads it without errors

Move SKILL.md, references/, and assets/ into skills/diagram-design/
and drop `"skills": ["./"]` from .claude-plugin/plugin.json.

Current state fails to load in Claude Code with:
    skills path escapes plugin directory: ./
    → Paths in plugin.json must not use ".." to reference files outside the plugin directory

Claude Code's plugin loader rejects `"./"` as a skill path because
it's ambiguous (plugin root vs. parent) and fails closed to prevent
path-escape attacks. The conventional layout is `skills/<name>/SKILL.md`
per plugin (compare obsidian-skills, claude-plugins-official).

README.md links are updated to point at the new paths so they keep
resolving on GitHub. SKILL.md's internal relative links (e.g.
`references/style-guide.md`) are unchanged — they still resolve
correctly from its new location.

Tested locally: plugin reloads cleanly with no errors and the skill
appears in /skills.

.claude-plugin/plugin.json

@@ -12,6 +12,5 @@
     "flowchart",
     "visualization",
     "editorial"
-  ],
-  "skills": ["./"]
+  ]
 }