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/02-ssh-auth-sock.fish

@@ -0,0 +1,9 @@
+# Create a predictable symlink to the current SSH auth socket.
+# tmux forces SSH_AUTH_SOCK to ~/.ssh/ssh_auth_sock globally
+# (see config/tmux/tmux.conf), so this symlink must exist for every
+# interactive shell -- not just bash login shells.
+set -l auth_sock "$HOME/.ssh/ssh_auth_sock"
+if set -q SSH_AUTH_SOCK; and test "$SSH_AUTH_SOCK" != "$auth_sock"; and test -S "$SSH_AUTH_SOCK"
+    mkdir -p "$HOME/.ssh"
+    ln -sf "$SSH_AUTH_SOCK" "$auth_sock"
+end