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

bash/bashrc-osx

@@ -38,9 +38,10 @@ fi
 #export LANG=en_US.UTF-8
 
 
-# Identity for the vim header; install.sh also writes it into ~/.gitconfig
-export USER=bchanot
-export EMAIL=bchanot@gmail.fr
+# Identity for git commits and vim headers. Filled by install.sh (asked once,
+# reused from an existing ~/.bashrc / ~/.zshrc on re-runs), never stored here.
+export USER="@USER@"
+export EMAIL="@EMAIL@"
 
 ## Activate and custom bash completion
 #bind 'TAB:menu-complete'