feat: add completions API and scanning logic

Implement a new completions system that scans markdown files for
dice notation and headings. The server now exposes a
`/__COMPLETIONS.json`
endpoint and automatically recomputes the completion index when
files are added, updated, or deleted.

src/cli/completions/index.ts

@@ -0,0 +1,32 @@
+/**
+ * Completion index — orchestrates all registered completion sources.
+ */
+
+import { diceSource } from "./sources/dice.js";
+import { linksSource } from "./sources/links.js";
+import type { CompletionSource, CompletionsPayload } from "./types.js";
+
+export type {
+  CompletionsPayload,
+  DiceCompletion,
+  LinkCompletion,
+} from "./types.js";
+
+/** Registered sources — open for extension */
+const sources: CompletionSource[] = [diceSource, linksSource];
+
+/**
+ * Scan the full content index and return structured completion data.
+ * Called at server startup and on any file change.
+ */
+export function scanCompletions(
+  index: Record<string, string>,
+): CompletionsPayload {
+  const payload: Record<string, unknown[]> = {};
+
+  for (const source of sources) {
+    payload[source.key] = source.scan(index);
+  }
+
+  return payload as unknown as CompletionsPayload;
+}