docs(copilot): add Copilot instructions for The Collective Hub

- Add comprehensive project overview and core philosophy
- Document file structure reference for the codebase
- Create key files reference table for task-specific guidance
- Include multi-tenant guidelines and site resolution flow

.github/instructions/svelte-ts.instructions.md

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+---
+description: 'Use when writing or editing .svelte.ts reactive state modules. Enforces Svelte 5 runes-based shared state patterns and forbids svelte/store primitives.'
+applyTo: '**/*.svelte.ts'
+---
+
+# Svelte Reactive Module Rules (`.svelte.ts`)
+
+Use `.svelte.ts` files for **global or cross-tree shared reactive state** that needs to be accessed outside of a component hierarchy. For state scoped to a component subtree, prefer `setContext` / `getContext` instead.
+
+## Module-Scope State
+
+Declare `$state` at module scope to create a reactive singleton. Export a factory function or a plain object with getter/setter methods — not the raw `$state` variable, which would allow callers to reassign it.
+
+```ts
+// ✅ src/lib/state/services.svelte.ts
+const _state = $state({ list: [] as Service[], loading: false });
+
+export const servicesStore = {
+	get list() {
+		return _state.list;
+	},
+	get loading() {
+		return _state.loading;
+	},
+	set(services: Service[]) {
+		_state.list = services;
+	},
+	setLoading(v: boolean) {
+		_state.loading = v;
+	}
+};
+```
+
+```ts
+// ❌ — callers can do `list = []` bypassing reactivity
+export let list = $state<Service[]>([]);
+```
+
+## Derived State
+
+Use `$derived` at module scope for computed values:
+
+```ts
+const _state = $state({ list: [] as Service[], search: '' });
+
+export const filtered = $derived(_state.list.filter((s) => s.name.includes(_state.search)));
+```
+
+## Never Use svelte/store
+
+Do not use `writable`, `readable`, or `derived` from `svelte/store` in new code.
+
+| Instead of               | Use                                            |
+| ------------------------ | ---------------------------------------------- |
+| `writable(value)`        | `$state(value)` in a `.svelte.ts` module       |
+| `derived(store, fn)`     | `$derived(fn)` in a `.svelte.ts` module        |
+| `readable(value, start)` | `$state` + a setup function in the module body |
+
+## When to Use `.svelte.ts` vs `setContext/getContext`
+
+| Scenario                                            | Pattern                                                              |
+| --------------------------------------------------- | -------------------------------------------------------------------- |
+| State needed across unrelated component trees       | `.svelte.ts` module singleton                                        |
+| State scoped to one subtree (e.g. a form, a widget) | `setContext` in parent, `getContext` in children                     |
+| State that needs SSR / per-request isolation        | `setContext` in `+layout.svelte` (never module singletons on server) |
+
+> **Warning**: Module singletons are shared across all users in SSR. Only use `.svelte.ts` singletons for state that is client-side only (e.g. UI state, client-cached data). For per-user server state, always use context.