Add bootstrap.sh with zsh and git modules

Cross-distro (RHEL/Debian) VM setup: zsh + oh-my-zsh with a self-healing
bash fallback for environments that reset the root filesystem or hardcode
bash for new terminals (e.g. Kasm Workspaces), plus git identity and
HTTPS credential-store configuration.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

zsh/zshrc

@@ -0,0 +1,16 @@
+# Managed by dotfiles/zsh/install.sh — edits made directly to ~/.zshrc will be
+# overwritten on the next bootstrap run. Edit zsh/zshrc in the dotfiles repo
+# instead.
+
+export ZSH="$HOME/.oh-my-zsh"
+
+ZSH_THEME="robbyrussell"
+
+plugins=(git)
+
+source "$ZSH/oh-my-zsh.sh"
+
+HISTSIZE=10000
+SAVEHIST=10000
+setopt SHARE_HISTORY
+setopt HIST_IGNORE_DUPS