refactor: remove implicit content sniffing and table conversion

- Split loadCSV into parseCSVString (content) and loadCSVFromPath
  (path); drop isCSV/looksLikeCsv heuristics
- Delete coerceSparkTables and markedTable label-header magic;
  plain markdown tables now render as plain tables
- Add explicit markdown role=spark-table fence syntax that converts
  pipe tables to CSV at scan time, with dice-header validation
- Map ESM-only github-slugger and csv-parse browser build to CJS in
  jest config; add content-registry tests

src/cli/completions/variable-system.test.ts

@@ -12,11 +12,8 @@
 // Mocks
 // ---------------------------------------------------------------------------
 
-jest.mock("csv-parse/browser/esm/sync", () => {
-  // Redirect browser-specific import to Node-compatible sync parser
-  const actual = jest.requireActual("csv-parse/sync");
-  return { parse: actual.parse };
-});
+// csv-parse/browser/esm/sync and github-slugger are mapped to CJS builds
+// globally in jest.config.js moduleNameMapper.
 
 jest.mock("github-slugger", () => {
   // Simple slugger mock for testing