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/sources/dice.ts

@@ -0,0 +1,38 @@
+/**
+ * Dice completion source — extracts `<md-dice>` text content from markdown files.
+ */
+
+import type { CompletionSource, DiceCompletion } from "../types.js";
+
+function looksLikeDice(raw: string): boolean {
+  if (raw.length > 80) return false;
+  return /^\d*d\d+/i.test(raw) || /^[+-]/.test(raw);
+}
+
+export const diceSource: CompletionSource = {
+  key: "dice",
+
+  scan(index) {
+    const items: DiceCompletion[] = [];
+    const tagRegex = /<md-dice[^>]*>\s*([\s\S]*?)\s*<\/md-dice>/gi;
+
+    for (const [path, content] of Object.entries(index)) {
+      if (!path.endsWith(".md")) continue;
+
+      let match: RegExpExecArray | null;
+      tagRegex.lastIndex = 0;
+      while ((match = tagRegex.exec(content)) !== null) {
+        const raw = match[1].trim();
+        if (!raw || !looksLikeDice(raw)) continue;
+
+        items.push({
+          label: raw,
+          notation: raw,
+          source: path,
+        });
+      }
+    }
+
+    return items;
+  },
+};