docs: update ABSTRACTIONS.md with _pinned_properties format

Document the "key:icon" string format used for _pinned_properties
in the system properties section and add it to the Type document
properties table.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

docs/ABSTRACTIONS.md

@@ -39,8 +39,9 @@ Any frontmatter field whose name starts with `_` is a **system property**:
 Examples:
 ```yaml
 _pinned_properties:       # which properties appear in the editor inline bar (per-type)
-  - key: status
-    icon: circle-dot
+  - "Status:circle-dot"   # format: "key:icon" (icon is optional)
+  - "Belongs to:arrow-up-right"
+  - "Due date:calendar"
 _icon: shapes             # icon assigned to a type
 _color: blue              # color assigned to a type
 _order: 10                # sort order in the sidebar
@@ -178,6 +179,7 @@ Each entity type can have a corresponding **type document** in the `type/` folde
 | `sort` | string | Default sort: "modified:desc", "title:asc", "property:Priority:asc" |
 | `view` | string | Default view mode: "all", "editor-list", "editor-only" |
 | `visible` | bool | Whether type appears in sidebar (default: true) |
+| `_pinned_properties` | list | Pinned properties shown in editor bar + note list. Each item: `"key:icon"` |
 
 **Type relationship**: When any entry has an `isA` value (e.g., "Project"), the Rust backend automatically adds a `"Type"` entry to its `relationships` map pointing to `[[type/project]]`. This makes the type navigable from the Inspector panel.