detectTownRoot() was only checking for mayor/town.json, but some
workspaces only have the mayor/ directory without town.json.
This caused mail routing to fail silently - messages showed
success but werent persisted because townRoot was empty.
Now uses workspace.Find() which supports both primary marker
(mayor/town.json) and secondary marker (mayor/ directory).
Fixes: gt-6v7z89
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>