Feat v0.8.5 extension composability (#72)

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

docs/PACKAGE-FORMAT.md

@@ -68,10 +68,66 @@ Only `manifest.json` is required. All other directories are optional and include
 | `db_tables` | Table definitions with columns and indexes |
 | `settings` | User-configurable settings with type, label, description, default |
 | `hooks` | Event bus subscription patterns |
-| `exports` | Functions exported by library packages |
+| `exports` | Functions exported for cross-package calls via `lib.require()` |
+| `depends` | Array of package IDs this package depends on |
+| `slots` | Named UI injection points (host surfaces declare these) |
+| `contributes` | Slot contributions this package injects into other surfaces |
 | `capabilities` | Environment requirements: `{"required": [...], "optional": [...]}` |
 | `schema_version` | Integer for additive schema migrations |
 
+## Composability: Slots and Contributions
+
+Packages compose with each other through named UI injection points (**slots**) and **contributions**.
+
+### Declaring Slots (Host Surfaces)
+
+Surfaces declare slots where other extensions can inject UI:
+
+```json
+{
+  "slots": {
+    "toolbar-actions": {
+      "description": "Toolbar action buttons",
+      "context": {
+        "noteId": "string — current note ID",
+        "getContent": "function — returns note body text"
+      }
+    }
+  }
+}
+```
+
+Slot names are namespaced at runtime as `{package-id}:{slot-name}` (e.g., `notes:toolbar-actions`). The `context` field documents what data the slot provides — it is not enforced at runtime.
+
+### Contributing to Slots
+
+Extensions declare which slots they inject into:
+
+```json
+{
+  "contributes": {
+    "notes:toolbar-actions": {
+      "label": "Dictate",
+      "icon": "🎤",
+      "description": "Voice-to-text dictation"
+    }
+  }
+}
+```
+
+Contributing extensions can be installed before or after their host surface — the coupling is soft. The admin can view all slots and their contributors at `GET /api/v1/admin/slots`.
+
+### Cross-Package Function Calls
+
+Any package that declares `exports` can be called via `lib.require()`, not just library-type packages. The caller must declare the target in its `depends` array:
+
+```json
+{
+  "depends": ["image-gen"],
+  "permissions": ["api.http"]
+}
+```
+
 ## Package Lifecycle
 
 1. **Install**: Upload a `.pkg` file via Admin > Packages or `POST /api/v1/admin/packages/install`. The kernel extracts the archive, creates database tables, and registers routes.