unify zsh+bash shell config; fix zsh startup

Support either shell as the login shell via stow selection (merges the
bashed branch in). shell/ holds shared POSIX env + interactive config;
each shell loads its own rc natively.

- zsh: add .zshenv (sets ZDOTDIR+env early), rename rc -> .zshrc, add
  login-only .zprofile (pyenv --path)
- bash: add ~/.bash_profile + ~/.bashrc stubs, .config/bash/{rc,prompt}
- shell: profile is POSIX env-only (no zsh-rc source); path rewritten
  POSIX + idempotent (flat ~/.local/bin, no subdir recursion); new
  interactive sources aliases/functions + pyenv shims
- drop dead shell/.zprofile symlink; move ZDOTDIR out of exports

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

zsh/.zshenv

@@ -0,0 +1,13 @@
+#!/bin/sh
+
+# Read first by zsh for EVERY shell (login or not), so this is the only place
+# ZDOTDIR can be set early enough to redirect where zsh looks for its rc files.
+export XDG_CONFIG_HOME="${XDG_CONFIG_HOME:-$HOME/.config}"
+export ZDOTDIR="$XDG_CONFIG_HOME/zsh"
+
+# Shared POSIX environment (idempotent):
+for _name in path exports; do
+	_file="$XDG_CONFIG_HOME/shell/$_name"
+	[ -r "$_file" ] && . "$_file"
+done
+unset _name _file