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>

shell/.config/shell/exports

@@ -7,8 +7,14 @@ export XDG_CACHE_HOME="$HOME/.cache"
 # Default editor
 export EDITOR='vim'
 
-# Shell history file
+# Shell history (env vars both shells read; each ignores the other's by name —
+# zsh uses SAVEHIST, bash uses HISTFILESIZE/HISTCONTROL. zsh dedup is a setopt,
+# not an env var, so it stays in .zshrc).
 export HISTFILE="${XDG_CACHE_HOME:-$HOME/.cache}/shell/history"
+export HISTSIZE=1000000        # commands kept in memory (both shells)
+export SAVEHIST=1000000        # zsh: lines written to $HISTFILE
+export HISTFILESIZE=1000000    # bash: lines written to $HISTFILE
+export HISTCONTROL=ignoreboth  # bash: drop dups + space-prefixed
 
 # Global git config file
 export GIT_CONFIG_GLOBAL="${XDG_CONFIG_HOME:-$HOME/.config}/git/config"

shell/.config/shell/interactive

@@ -8,5 +8,9 @@ for _file in "${XDG_CONFIG_HOME:-$HOME/.config}"/shell/{aliases,functions,extra}
 done
 unset _file
 
+# Ensure the history dir exists; neither shell mkdir's it, so without this
+# $HISTFILE writes fail silently on a fresh host.
+[ -n "$HISTFILE" ] && mkdir -p "$(dirname "$HISTFILE")"
+
 # pyenv interactive shims (pyenv init - autodetects the running shell):
 command -v pyenv >/dev/null 2>&1 && eval "$(pyenv init -)"