feat: add stat completion source

Implement a new completion source that extracts stat definitions
from markdown files using `yaml` or `csv` code blocks with
`role=stat`.

src/cli/completions/index.ts

@@ -5,6 +5,7 @@
 import { diceSource } from "./sources/dice.js";
 import { linksSource } from "./sources/links.js";
 import { sparkTablesSource } from "./sources/spark-tables.js";
+import { statsSource } from "./sources/stats.js";
 import type { CompletionSource, CompletionsPayload } from "./types.js";
 
 export type {
@@ -12,6 +13,7 @@ export type {
   DiceCompletion,
   LinkCompletion,
   SparkTableCompletion,
+  StatDef,
 } from "./types.js";
 
 /** Registered sources — open for extension */
@@ -19,6 +21,7 @@ const sources: CompletionSource[] = [
   diceSource,
   linksSource,
   sparkTablesSource,
+  statsSource,
 ];
 
 /**