feat(tmux): auto-name sessions with emoji + codename

- scripts/tmux-session-name.sh: generates random emoji + adj-noun name
  40 emojis x 40 adjectives x 40 nouns = 64,000 combinations
  Only renames numerically-named sessions (skips explicit names)

- tmux.conf.local: session-created hook calls the script automatically

- shell/common: added DOTFILES=$HOME/dotfiles env var so hooks and
  scripts can find the dotfiles root without hardcoded paths

Examples: '🐉 sonic-oracle', '🌟 electric-nebula', '🔱 nova-comet'

shell/.config/shell/common

@@ -5,6 +5,9 @@
 export EDITOR='nvim'
 export VISUAL='nvim'
 
+# Dotfiles root — used by scripts and tmux hooks
+export DOTFILES="$HOME/dotfiles"
+
 # Man page rendering
 export MANROFFOPT="-c"