Feat v0.8.5 extension composability (#72)

Co-authored-by: Jeffrey Smith <jasafpro@gmail.com>
Co-committed-by: Jeffrey Smith <jasafpro@gmail.com>

docs/EXTENSION-GUIDE.md

@@ -77,7 +77,10 @@ Every package has a `manifest.json` at its root. Example for a surface:
 | `api_schema` | no | OpenAPI documentation for extension API routes (see below) |
 | `db_tables` | no | Table definitions (see below) |
 | `settings` | no | User-configurable settings schema |
-| `exports` | libraries | Functions exported for other packages |
+| `exports` | no | Functions exported for cross-package calls via `lib.require()` |
+| `depends` | no | Array of package IDs this package depends on |
+| `slots` | no | Named UI injection points for host surfaces |
+| `contributes` | no | Slot contributions into other surfaces |
 | `hooks` | no | Event bus subscriptions |
 | `config_section` | no | Settings/Admin panel injection (see below) |
 | `capabilities` | no | Environment requirements (see below) |
@@ -194,6 +197,117 @@ sandbox permission model.
 In Starlark, check permissions inline via `req["permissions"]` or call
 `permissions.check(user_id, "image-gen.use")`.
 
+## Extension Composability
+
+Extensions compose with each other through three mechanisms: manifest-declared
+**slots** (UI injection points), **contributions** (UI components injected into
+those slots), and cross-package **function calls** via `lib.require()`.
+
+### Slots — Host Surfaces Declare Injection Points
+
+A surface declares named slots in its manifest where other extensions can
+inject UI components:
+
+```json
+{
+  "id": "notes",
+  "type": "surface",
+  "slots": {
+    "toolbar-actions": {
+      "description": "Toolbar action buttons",
+      "context": {
+        "noteId": "string",
+        "getContent": "function — returns note body",
+        "setContent": "function — replaces note body"
+      }
+    }
+  }
+}
+```
+
+The surface renders slot contents using the SDK helper:
+
+```javascript
+html`<div class="toolbar">
+    ${sw.slots.renderAll('notes:toolbar-actions', {
+        noteId: note.id,
+        getContent: () => editor.getValue(),
+        setContent: (text) => editor.setValue(text),
+    })}
+</div>`
+```
+
+### Contributions — Extensions Inject UI
+
+An extension declares which slots it contributes to:
+
+```json
+{
+  "id": "note-dictate",
+  "type": "extension",
+  "contributes": {
+    "notes:toolbar-actions": {
+      "label": "Dictate",
+      "icon": "🎤",
+      "description": "Voice-to-text dictation"
+    }
+  }
+}
+```
+
+In its JavaScript, it registers the component:
+
+```javascript
+sw.slots.register('notes:toolbar-actions', {
+    id: 'note-dictate',
+    priority: 200,
+    component: ({ noteId, setContent, getContent }) => {
+        // ... component implementation
+        return html`<button class="sw-btn sw-btn--ghost">🎤</button>`;
+    }
+});
+```
+
+Contributions are soft-coupled — install order doesn't matter. The admin
+can view all slots and contributors at **Admin > Packages** or via
+`GET /api/v1/admin/slots`.
+
+### Cross-Package Function Calls
+
+Any package that declares `exports` in its manifest can be called by other
+packages via `lib.require()`. The caller declares the dependency:
+
+```json
+{
+  "id": "note-ai",
+  "depends": ["llm-bridge"],
+  "permissions": ["api.http"]
+}
+```
+
+In Starlark:
+
+```python
+llm = lib.require("llm-bridge")
+result = llm.complete([{"role": "user", "content": prompt}])
+```
+
+The called function runs with the *target* package's permissions, not the
+caller's. This is the same security model as library packages.
+
+### Slot Naming Convention
+
+Slot names follow `{host-package-id}:{slot-name}`. The colon separates the
+namespace from the slot. Standard slots for first-party packages:
+
+| Slot | Host | Use Case |
+|------|------|----------|
+| `notes:toolbar-actions` | notes | Dictation, AI tools, formatting |
+| `notes:note-footer` | notes | Related items, AI summary |
+| `chat:composer-tools` | chat | Image gen, file attach |
+| `chat:message-actions` | chat | Reactions, translate, bookmark |
+| `chat:image-actions` | chat | Regen, edit, upscale |
+
 ## Starlark Sandbox API
 
 Starlark scripts run server-side with a 1M operation budget and no