Promote AgentCompany-New to the repository root and retire the old app

The legacy application - its src, tests, landing site, promo assets, docs,
release tooling, CI workflows, and root configs - leaves the tree; it stays
reachable in history. Everything under AgentCompany-New moves up unchanged:
same package name, so the production userData/database path is untouched,
and e2e/build scripts already resolved paths relative to the project. The
developer cred.txt stays gitignored at its new location. Full verify:ci
passes from the new root.

src/shared/mention-handles.ts

@@ -0,0 +1,11 @@
+export const RESERVED_MENTION_HANDLES = new Set(["board", "oncall"]);
+
+export function canonicalMentionHandle(value: string): string {
+  return value.toLowerCase().replace(/[^a-z0-9]/g, "");
+}
+
+export function assertUsableMentionName(value: string): void {
+  if (RESERVED_MENTION_HANDLES.has(canonicalMentionHandle(value))) {
+    throw new Error("Mention handle is reserved.");
+  }
+}