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/00-setup.fish

@@ -0,0 +1,20 @@
+if command -q mise
+    mise activate fish | source
+end
+
+if command -q direnv
+    direnv hook fish | source
+end
+
+if command -q zoxide;
+    zoxide init fish | source
+end
+
+if command -q fzf
+   fzf --fish | source
+end
+
+if command -q jj
+    jj util completion fish | source
+end
+