docs: document types-as-files architecture

Add documentation for type documents, the Type relationship, and
the UI behavior changes (pinned type docs in section groups,
Instances group in entity view, clickable Type chip in Inspector).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

docs/ABSTRACTIONS.md

@@ -35,6 +35,7 @@ Entity type is inferred from the folder structure. The vault is organized by typ
 
 ```
 ~/Laputa/
+├── type/           → "Type"       ← type definition documents
 ├── project/        → "Project"
 ├── responsibility/ → "Responsibility"
 ├── procedure/      → "Procedure"
@@ -51,6 +52,22 @@ Entity type is inferred from the folder structure. The vault is organized by typ
 
 Mapping logic lives in `vault.rs:parse_md_file()`. If a folder doesn't match any known type, the folder name is capitalized and used as-is.
 
+### Types as Files
+
+Each entity type can have a corresponding **type document** in the `type/` folder (e.g., `type/project.md`, `type/person.md`). Type documents:
+
+- Have `Is A: Type` in their frontmatter
+- Describe what the type means, its expected properties, and how it relates to other types
+- Are navigable entities — they appear in the sidebar under "Types" and can be opened/edited like any other note
+- Serve as the "definition" for their type category
+
+**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.
+
+**UI behavior**:
+- Clicking a section group header (e.g., "Projects") pins the type document at the top of the NoteList if it exists, with instances listed below
+- Viewing a type document in entity view shows an "Instances" group listing all entries of that type
+- The Type field in the Inspector properties panel is rendered as a clickable chip that navigates to the type document
+
 ### Frontmatter Format
 
 Standard YAML frontmatter between `---` delimiters: