refactor(shell): modularize shell config into per-shell profiles and conf.d

Split bash config into dedicated aliases and profile files. Extract
fish shell init from config.fish into composable conf.d fragments
(setup, environment, ssh-auth-sock). Remove the generic profile in
favor of the bash-specific one. Add a static SSH_AUTH_SOCK symlink
so tmux sessions always see the current socket regardless of which
shell spawned them.

config/fish/conf.d/01-environment.fish

@@ -0,0 +1,11 @@
+fish_add_path "$HOME/.local/bin"
+fish_add_path "$HOME/.cargo/bin"
+
+set -gx npm_config_prefix "$HOME/.local"
+
+if command -q nvim 
+    set -gx EDITOR nvim
+end
+
+set -gx VISUAL $EDITOR
+