Rewrite README as a short project front page; add a Nix flake

The README is now a landing page: the idea (a mesh VPN with no server, joined
with a network name and a secret), install and platform tables, exit nodes,
and a short build section. Implementation detail stays in docs/.

The flake builds tsng and the tray from source and ships a NixOS module
(services.tsunagi). Dist install notes now show the real `join -n -s` syntax,
and two links that pointed into the old README are redirected.

Co-Authored-By: Claude <noreply@anthropic.com>

dist/macos/README.md

@@ -15,7 +15,7 @@ socket and do **not** need `sudo`:
 
 ```sh
 tsng status
-tsng join <network-name> <tsn1…secret>
+tsng join -n <network-name> -s <tsn1…secret>
 ```
 
 The agent uses fixed system paths, so the commands find it regardless of whose