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/selection-managers.md

@@ -0,0 +1,93 @@
+# Selection Managers
+
+*Two-layer selection architecture: viewer manager (hierarchy) + editor manager (phase-aware).*
+
+Applies to: `packages/viewer/src/components/viewer/selection-manager.tsx`, `apps/editor/components/editor/selection-manager.tsx`.
+
+There are two selection managers. They are separate components, not the same component configured differently.
+
+| Component | Location | Knows about |
+|---|---|---|
+| `SelectionManager` | `packages/viewer/src/components/viewer/selection-manager.tsx` | Viewer state only |
+| `SelectionManager` (editor) | `apps/editor/components/editor/selection-manager.tsx` | Phase, mode, tool state |
+
+The viewer's manager is the default. The editor mounts its own manager as a child of `<Viewer>`, overriding the default behaviour via the viewer-isolation pattern.
+
+---
+
+## How Selection Works
+
+**Event flow:**
+
+```
+useNodeEvents(node, type) on a renderer mesh
+  → emitter.emit('wall:click', NodeEvent)
+  → SelectionManager listens via emitter.on(…)
+  → calls useViewer.setSelection(…)
+  → outliner sync re-runs → Three.js outline updates
+```
+
+`useNodeEvents` returns R3F pointer handlers. Spread them onto the mesh:
+
+```tsx
+const events = useNodeEvents(node, 'wall')
+return <mesh ref={ref} {...events} />
+```
+
+Events are suppressed during camera drag (`useViewer.getState().cameraDragging`).
+
+---
+
+## Viewer Selection Manager
+
+Hierarchical path: **Building → Level → Zone → Elements**
+
+At each level, only the next tier is selectable. Clicking outside deselects. The path is stored in `useViewer`:
+
+```ts
+type SelectionPath = {
+  buildingId: string | null
+  levelId: string | null
+  zoneId: string | null
+  selectedIds: string[]   // walls, items, slabs, etc.
+}
+```
+
+`setSelection` has a hierarchy guard: setting `levelId` without `buildingId` resets children. Use `resetSelection()` to clear everything.
+
+Multi-select: `Ctrl/Meta + click` toggles an ID in `selectedIds`. Regular click replaces it.
+
+---
+
+## Editor Selection Manager
+
+Extends selection with phase awareness from `useEditor`. The viewer's `SelectionManager` is **not** mounted in the editor; this one takes its place (injected as a child of `<Viewer>`).
+
+```
+phase: 'site'      → selectable: buildings
+phase: 'structure' → selectable: walls, zones, slabs, ceilings, roofs, doors, windows
+  structureLayer: 'zones'    → only zones
+  structureLayer: 'elements' → all structure types
+phase: 'furnish'   → selectable: furniture items only
+```
+
+Clicking a node of a different phase auto-switches the phase. Double-click drills into a context level.
+
+---
+
+## Rules
+
+- **Never add selection logic to renderers.** Renderers spread `useNodeEvents` events and stop there. All selection decisions live in the selection manager.
+- **Never add editor phase logic to the viewer's SelectionManager.** Phase, mode, and tool awareness belong exclusively in the editor's selection manager.
+- **`useViewer` is the single source of truth for selection state.** Both managers read and write through `setSelection` / `resetSelection`. Nothing else should mutate `selection` directly.
+- **Outliner arrays are mutated in-place** (not replaced) for performance. Don't assign new arrays to `outliner.selectedObjects` or `outliner.hoveredObjects`.
+- **Hover is a separate scalar** (`hoveredId: string | null`), not part of `selectedIds`. Update it via `setHoveredId`.
+
+---
+
+## Adding Selectability to a New Node Type
+
+1. Add the type to `SelectableNodeType` in the viewer store / selection manager.
+2. Make sure its renderer calls `useNodeEvents(node, type)` and spreads the handlers.
+3. Add a case to whichever selection strategy needs it (viewer hierarchy level or editor phase).
+4. Ensure `useRegistry` is called in the renderer so the outliner can highlight it.