refactor: trim Diagnostic to fields produced this slice (task 0003)

Drop the Point/Position types and the source/position fields from
Diagnostic. Nothing constructs a diagnostic yet — diagnostics is always
empty until task 0007 emits the first invalid-frontmatter warning, which
adds the locator fields back additively alongside its own tests.

packages/core/src/types.ts

@@ -2,23 +2,10 @@
 
 export type Frontmatter = Record<string, unknown>;
 
-export interface Point {
-  line: number;
-  column: number;
-  offset?: number;
-}
-
-export interface Position {
-  start: Point;
-  end: Point;
-}
-
 export interface Diagnostic {
   severity: "warning";
   code: string;
   message: string;
-  source?: string;
-  position?: Position;
 }
 
 export interface TextNode {