Restructure agent config: unified .agents/skills + wiki/architecture

Replace per-tool rule trees (.cursor/rules, .claude/rules, .codex/rules)
with a single wiki/architecture/ source — 11 pages + README — readable
as plain markdown by any agent. Canonical skills live in .agents/skills/;
.claude/skills, .cursor/skills, .codex/skills are directory symlinks.

AGENTS.md is the entrypoint (rewritten as a lean overview, no per-tool
path lists). CLAUDE.md, GEMINI.md, and .github/copilot-instructions.md
all point to it.

Add open-pr skill (uses .github/pull_request_template.md as the source
of truth for the PR body) and remove the dangling .claude/CLAUDE.md
relative symlink.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

wiki/architecture/renderers.md

@@ -0,0 +1,66 @@
+# Renderers
+
+*Node renderer pattern in `packages/viewer`.*
+
+Applies to: `packages/viewer/**`.
+
+Renderers live in `packages/viewer/src/components/renderers/`. Each renderer is responsible for one node type's Three.js geometry and materials — nothing else.
+
+## Dispatch Chain
+
+```
+<SceneRenderer>          — iterates rootNodeIds from useScene
+  └─ <NodeRenderer>      — switches on node.type, renders the matching component
+       └─ <WallRenderer> — (or SlabRenderer, DoorRenderer, …)
+```
+
+See `packages/viewer/src/components/renderers/scene-renderer.tsx` and `packages/viewer/src/components/renderers/node-renderer.tsx`.
+
+## Renderer Responsibilities
+
+A renderer **should**:
+- Read its node from `useScene` via the node's ID
+- Register its mesh(es) with `useRegistry()` so other systems can look them up
+- Subscribe to pointer events via `useNodeEvents()`
+- Render geometry and apply materials based on node properties
+
+A renderer **must not**:
+- Run geometry generation logic (that belongs in a System)
+- Import anything from `apps/editor`
+- Manage selection state directly (use `useViewer` for read, emit events for write)
+- Perform expensive per-frame calculations in the component body
+
+## Example — Minimal Renderer
+
+```tsx
+// packages/viewer/src/components/renderers/my-node/index.tsx
+import { useRegistry } from '@pascal-app/core'
+import { useNodeEvents } from '../../hooks/use-node-events'
+import { useScene } from '@pascal-app/core'
+
+export function MyNodeRenderer({ node }: { node: MyNode }) {
+  const ref = useRef<Mesh>(null!)
+  useRegistry(node.id, 'my-node', ref)   // 3 args: id, type, ref — no return value
+  const events = useNodeEvents(node, 'my-node')
+
+  return (
+    <mesh ref={ref} {...events}>
+      <boxGeometry args={[node.width, node.height, node.depth]} />
+      <meshStandardMaterial color={node.color} />
+    </mesh>
+  )
+}
+```
+
+## Adding a New Node Type
+
+1. Create `packages/viewer/src/components/renderers/<type>/index.tsx`
+2. Add a case to `NodeRenderer` in `node-renderer.tsx`
+3. Add the corresponding system in `packages/core/src/systems/` if the node needs derived geometry
+4. Export from `packages/viewer/src/index.ts` if needed externally
+
+## Performance Notes
+
+- Use `useMemo` for geometry that depends on node properties — avoid recreating on every render.
+- For complex cutout or boolean geometry, delegate to a System (e.g. `WallCutout`).
+- Register one mesh per node ID; if a renderer spawns multiple meshes, use a group ref or pick the primary one for registry.