feat: add ReactiveStatManager for markdown template support

Introduce ReactiveStatManager to allow dynamic stat replacement within
rendered markdown content. This component scans the article DOM for
`${key}` patterns and reactively updates them based on the journal
stream.

As part of this change, the SVG-based StatSheet system is removed in
favor of this more flexible text-based approach.

src/cli/completions/block-processor.ts

@@ -20,7 +20,6 @@ import {
   parseBlockAttrs,
   resolveBlockAs,
 } from "./block-scanner.js";
-import type { StatSheet } from "./types.js";
 
 // Re-export shared pieces for convenience
 export {
@@ -37,7 +36,6 @@ export {
 export interface ProcessedBlocks {
   stats: StatDef[];
   statTemplates: StatTemplate[];
-  statSheets: StatSheet[];
 }
 
 export interface BlockResult {
@@ -78,7 +76,6 @@ export function processBlocks(
   const blocks: ProcessedBlocks = {
     stats: [],
     statTemplates: [],
-    statSheets: [],
   };
 
   const stripped = content.replace(