feat(cli): Add tttk check validator

Statically validate a directory of tttk documents and report structured diagnostics with file/line positions, exit codes, and --json output. Checks cover code-block roles, unresolved content references, tag blocks, element names, links, anchors, and assets. Runs over the same content registry as the app.

src/cli/commands/check.ts

@@ -0,0 +1,23 @@
+import type { CheckCommandHandler } from "../types.js";
+import { formatHuman, formatJson } from "../validate/diagnostics.js";
+import { runCheck } from "../validate/index.js";
+
+export const checkCommand: CheckCommandHandler = async (dir, options) => {
+  let report;
+  try {
+    report = runCheck(dir);
+  } catch (error) {
+    console.error(error instanceof Error ? error.message : String(error));
+    process.exitCode = 2;
+    return;
+  }
+
+  console.log(
+    options.json ? formatJson(report) : formatHuman(report, options.quiet),
+  );
+
+  const failed =
+    report.summary.errors > 0 ||
+    (options.strict && report.summary.warnings > 0);
+  if (failed) process.exitCode = 1;
+};