update CLAUDE.md philosophy, set fish as default shell, improve tmux config

CLAUDE.md: add nix-only rule, verify-after-change rule, teach-debugging rule.
darwin: set fish as default user shell.
tmux: configure better-mouse-mode, fix catppuccin status, add extended keys.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

home-manager/modules/tmux.nix

@@ -23,14 +23,23 @@
     mouse = true;
     prefix = "C-a"; # More ergonomic than C-b
     escapeTime = 0; # No delay for escape (important for vim)
-    sensibleOnTop = true;
+    sensibleOnTop = false; # sensible's reattach-to-user-namespace overrides default-shell on macOS
+    shell = "${pkgs.fish}/bin/fish";
     
     plugins = with pkgs.tmuxPlugins; [
       # Core essentials
       sensible
       yank # System clipboard integration
       vim-tmux-navigator # Seamless vim/tmux pane navigation
-      better-mouse-mode # Enhanced mouse support
+      {
+        plugin = better-mouse-mode;
+        extraConfig = ''
+          set -g @scroll-without-changing-pane 'on'
+          set -g @scroll-in-moused-over-pane 'on'
+          set -g @emulate-scroll-for-no-mouse-alternate-buffer 'on'
+          set -g @scroll-speed-num-lines-per-scroll '3'
+        '';
+      }
       
       # Session persistence - survive reboots
       {
@@ -82,7 +91,7 @@
           
           # Status bar modules
           set -g @catppuccin_status_modules_right 'session date_time'
-          set -g @catppuccin_status_modules_left ''''
+          set -g @catppuccin_status_modules_left ""
           set -g @catppuccin_date_time_text '%H:%M'
         '';
       }
@@ -93,9 +102,16 @@
       # GENERAL SETTINGS
       # ============================================
       
+      # Force fish as the shell — override sensible plugin's reattach-to-user-namespace
+      set -g default-command "${pkgs.fish}/bin/fish"
+
       # True color support
       set -ag terminal-overrides ",xterm-256color:RGB"
       set -ag terminal-overrides ",*256col*:Tc"
+
+      # Extended keys — lets tmux forward Shift+Enter, Ctrl+Enter, etc.
+      set -s extended-keys on
+      set -as terminal-features 'xterm*:extkeys'
       
       # Undercurl support (for spell checking in nvim)
       set -as terminal-overrides ',*:Smulx=\E[4::%p1%dm'