Add bin/ module: userland CLI tools into ~/.local/bin

New bin/install.sh downloads self-contained release binaries (ripgrep, fd,
bat, fzf, jq) into ~/.local/bin — no sudo, nothing outside $HOME, so they
persist across Kasm redeploys with no reinstall. Statically linked where
available, so they keep working even after the root fs is wiped. Idempotent:
pinned versions already present are skipped.

lib/fetch.sh adds arch detection + tarball/raw binary download helpers.
zsh/zshrc puts ~/.local/bin on PATH; bootstrap.sh runs the module.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

zsh/zshrc

@@ -2,6 +2,12 @@
 # overwritten on the next bootstrap run. Edit zsh/zshrc in the dotfiles repo
 # instead.
 
+# Userland tools installed by dotfiles/bin/install.sh live here.
+case ":$PATH:" in
+  *":$HOME/.local/bin:"*) ;;
+  *) export PATH="$HOME/.local/bin:$PATH" ;;
+esac
+
 export ZSH="$HOME/.oh-my-zsh"
 
 ZSH_THEME="robbyrussell"