fix: strip all accumulated tags in stripTagPrefix (#12)

Defensive fix: change TAG_PREFIX_REGEX from /^§\d+§\s*/ to /^(?:§\d+§\s*)+/ so stripTagPrefix removes all consecutive tag prefixes, not just the outermost one. In practice tags don't accumulate because prependTag always strips before prepending, but this makes the function robust against any future path that might double-tag content.

Thanks to @tomolom for the contribution.

Fixes #11

Author: tomolom

packages/plugin/src/hooks/magic-context/tag-content-primitives.ts

@@ -1,7 +1,7 @@
 import type { ThinkingLikePart } from "./tag-messages";
 
 const encoder = new TextEncoder();
-const TAG_PREFIX_REGEX = /^§\d+§\s*/;
+const TAG_PREFIX_REGEX = /^(?:§\d+§\s*)+/;
 
 export function byteSize(value: string): number {
     return encoder.encode(value).length;