docs: unify product naming — diagram/diagram-design instead of Schematic (#50)

Rename remaining "Schematic" references to diagram / diagram-design / Diagram Design across SKILL.md, onboarding.md, and style-guide.md. Rebased onto current main and bumped both plugin manifests to 2.3.5. Thanks @philyuchkoff.

.codex-plugin/plugin.json

@@ -1,7 +1,7 @@
 {
   "name": "diagram-design",
   "description": "Create branded architecture, IT current-state, flowchart, sequence, state machine, ER/data model, timeline, swimlane, quadrant, radar/spider, loop/flywheel, nested, tree, org chart, layer stack, Venn, pyramid/funnel, bar, line, Gantt and scatter charts, high-level, process, medallion, data flow, DP integration, or DP security matrix diagrams as standalone HTML/SVG/PNG. Redraw .drawio/.drawio.png/.drawio.svg or Mermaid .mmd sources at a chosen size/detail; onboard brand tokens from a website; add semantic patterns, callouts, accessible motion, or sketchy/hand-drawn styling.",
-  "version": "2.3.4",
+  "version": "2.3.5",
   "author": {
     "name": "Cathryn Lavery",
     "url": "https://github.com/cathrynlavery"