shell: centralize history config and create history dir

Move HISTFILE/HISTSIZE/SAVEHIST/HISTFILESIZE/HISTCONTROL into the shared
shell/exports as env vars; each shell ignores the other's name-specific
ones. Dedup that can't be an env var stays per-shell: HISTCONTROL=ignoreboth
for bash, setopt hist_ignore_all_dups/hist_ignore_space for zsh.

Also mkdir -p the HISTFILE parent in shell/interactive, so a fresh host
doesn't silently drop history when ~/.cache/shell is absent.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

zsh/.config/zsh/.zshrc

@@ -12,9 +12,9 @@ setopt autocd		# Automatically cd into typed directory.
 stty stop undef		# Disable ctrl-s to freeze terminal.
 setopt interactive_comments
 
-# History in cache directory:
-HISTSIZE=10000000
-SAVEHIST=10000000
+# HISTFILE/HISTSIZE/SAVEHIST come from ~/.config/shell/exports (shared).
+# Dedup is a setopt, not an env var (bash's HISTCONTROL=ignoreboth equivalent):
+setopt hist_ignore_all_dups hist_ignore_space
 
 # Basic auto/tab complete:
 autoload -U compinit