feat: auto-detect and install missing shell frameworks after restore

After mackup restores dotfiles, ~/.zshrc may reference tools (Oh My Zsh,
nvm, pyenv, rustup, …) that haven't been installed yet, causing errors on
first shell login.

lib/shell.sh:
  - Defines 13 built-in frameworks with detect + install commands
  - Scans ~/.zshrc / ~/.bashrc etc. for references before installing
    (avoids installing things the user doesn't actually use)
  - SSR_SHELL_DEPS_EXTRA in ssr.conf for custom user additions

bin/ssr-restore:
  - Adds `shell_deps` as a resumable step after mackup_restore
  - Sources ~/.config/ssr/post-restore.sh if present (user hook)

config/post-restore.sh.example:
  - Template with commented examples (OMZ plugins, npm globals, SSH keys)

Co-authored-by: Cursor <cursoragent@cursor.com>

config/post-restore.sh.example

@@ -0,0 +1,60 @@
+#!/usr/bin/env bash
+# config/post-restore.sh — custom post-restore hooks.
+#
+# Copy this file to ~/.config/ssr/post-restore.sh and add your own steps.
+# It is sourced by `ssr restore` after all built-in steps complete.
+#
+# Available helpers (from lib/common.sh):
+#   ssr::log  "message"   — info line
+#   ssr::ok   "message"   — success line
+#   ssr::warn "message"   — warning line (non-fatal)
+#   ssr::confirm "prompt" — interactive y/N prompt
+#
+# Available variables:
+#   SSR_ROOT       — path to the ssr repository
+#   backup_dir     — cloud backup directory for this host
+#   mackup_dir     — cloud Mackup directory
+#
+# EXAMPLES
+# -------------------------------------------------------------------------
+
+# --- Custom shell dep (extends SSR_SHELL_DEPS_EXTRA in ssr.conf) ---------
+# If you prefer the hook-file approach over ssr.conf arrays, add deps here:
+#
+# if [[ ! -d "$HOME/.my-custom-tool" ]]; then
+#     ssr::log "Installing my-custom-tool..."
+#     curl -sSL https://example.com/install.sh | bash
+# fi
+
+# --- Oh My Zsh plugins not installed via Homebrew ------------------------
+# Plugins are usually git-cloned into ${ZSH_CUSTOM}/plugins.
+# Example: zsh-syntax-highlighting
+#
+# ZSH_CUSTOM="${ZSH_CUSTOM:-$HOME/.oh-my-zsh/custom}"
+# if [[ -d "$HOME/.oh-my-zsh" && ! -d "$ZSH_CUSTOM/plugins/zsh-syntax-highlighting" ]]; then
+#     git clone https://github.com/zsh-users/zsh-syntax-highlighting.git \
+#         "$ZSH_CUSTOM/plugins/zsh-syntax-highlighting"
+# fi
+
+# --- npm global packages --------------------------------------------------
+# npm globals are not tracked by Homebrew; restore them here.
+#
+# if command -v npm >/dev/null 2>&1; then
+#     npm install -g typescript prettier eslint
+# fi
+
+# --- macOS app configuration not covered by Mackup -----------------------
+# Example: Dock position
+#
+# defaults write com.apple.dock orientation -string left
+# killall Dock
+
+# --- SSH key import -------------------------------------------------------
+# Keys are intentionally excluded from Mackup (they are sensitive).
+# Restore from an encrypted backup or password manager here:
+#
+# op item get "SSH Key (personal)" --fields "private key" > "$HOME/.ssh/id_ed25519"
+# chmod 600 "$HOME/.ssh/id_ed25519"
+
+# --- GPG key import -------------------------------------------------------
+# gpg --import /path/to/exported.gpg