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>

shell/.config/shell/profile

@@ -1,17 +1,13 @@
 #!/bin/sh
 
-# Load the shell dotfiles, and then some:
-# * ~/.path can be used to extend `$PATH`.
-# * ~/.extra can be used for other settings you don’t want to commit.
-for file in ~/.config/shell/{path,exports,aliases,functions,extra}; do
-        [ -r "$file" ] && [ -f "$file" ] && source "$file";
-done;
-unset file;
-
-source ~/.config/zsh/rc
-
-# Setup pyenv, if it is installed:
-if [ -x "$(command -v pyenv)" ]; then
-	eval "$(pyenv init --path)"
-	eval "$(pyenv init -)"
-fi
+# POSIX login environment. Sourced directly by a /bin/sh login (display
+# managers, plain sh) and indirectly by bash via ~/.bashrc. Env only — no
+# brace expansion (dash doesn't expand it), no shell-specific rc.
+#
+# Interactive config (aliases, functions, prompt) is loaded by each shell's
+# own rc via ~/.config/shell/interactive, not here.
+for _name in path exports; do
+	_file="${XDG_CONFIG_HOME:-$HOME/.config}/shell/$_name"
+	[ -r "$_file" ] && . "$_file"
+done
+unset _name _file