feat(identity): ask name/email at install, no hardcoded USER/EMAIL in the repo

- rc templates carry @USER@/@EMAIL@ placeholders; install.sh renders them
- resolve_identity: an export already in ~/.bashrc or ~/.zshrc is reused silently,
  else IDENTITY_USER/IDENTITY_EMAIL, else a prompt, else login name + empty email
- render_gitconfig generalised to render_identity_template (gitconfig unchanged)
- vimrc reads g:_author/g:_email from $USER/$EMAIL instead of hardcoded values
- README + CLAUDE.md

vim/vimrc

@@ -1,9 +1,9 @@
 " Uset compatibility with Vii
 set nocompatible
 
-" add user info variable
-let g:_author = "Bastien Chanot"
-let g:_email = "chanot.bastien@gmail.com"
+" user info from the shell identity (USER / EMAIL exported by the deployed rc)
+let g:_author = $USER
+let g:_email = exists('$EMAIL') ? $EMAIL : ''
 
 " Enable pathogen for plugins
 execute pathogen#infect()