feat: add bin/setup bootstrap script and update README installation docs

- bin/setup: curl-pipeable installer — re-execs with bash, detects OS,
  installs git (apt/xcode-select), clones dotfiles into ~, runs arty deps,
  installs arty zsh completion, prints next steps; fully idempotent
- README.md: replace Quick Start with Installation one-liner, add step table,
  After Installation and Manual Setup sections; fix stale ~/scripts reference
- .gitignore: whitelist bin/setup

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

README.md

@@ -46,18 +46,53 @@
 
 ---
 
-## QUICK START RITUAL
+## INSTALLATION
 
-### Summoning the Environment
+### One-liner bootstrap
 
 ```bash
-# Clone this unholy repository
-git init && git remote add origin ssh://git@dev.pivoine.art/valknar/home.git
-git fetch && git reset --hard origin/main
-git branch --set-upstream-to=origin/main main
+curl https://dev.pivoine.art/valknar/home/raw/branch/main/bin/setup -sSf | sh
+```
 
-# Clone scripts repository
-git clone ssh://git@dev.pivoine.art/valknar/scripts.git ~/scripts
+**Prerequisites:** `curl` or `wget` · `sh`/`bash` · `sudo` *(only needed on Debian/Ubuntu if git is missing)*
+
+### What the setup script does
+
+| Step | Action |
+|------|--------|
+| 1 | Detects OS (Debian/Ubuntu, macOS, other Linux) |
+| 2 | Installs `git` if missing — `apt-get` on Debian/Ubuntu, `xcode-select` on macOS |
+| 3 | Clones this repo into `~` via HTTPS — or `git pull` if already present *(idempotent)* |
+| 4 | Runs `arty deps` — clones nvm, rbenv, pyenv, oh-my-zsh, powerlevel10k, zsh plugins |
+| 5 | Installs `arty` zsh completion to `~/.zsh/completions/_arty` |
+| 6 | Prints next steps |
+
+### After installation
+
+```bash
+exec zsh                  # start a new shell session
+
+p10k configure            # first time: configure the Powerlevel10k prompt
+
+arty ssh:keygen           # generate an SSH key, then add it at https://dev.pivoine.art
+
+# switch the dotfiles remote from HTTPS to SSH (after key is set up)
+git -C ~ remote set-url origin ssh://git@dev.pivoine.art/valknar/home.git
+```
+
+---
+
+## MANUAL SETUP
+
+> The one-liner above handles all of this automatically.
+
+```bash
+cd ~
+git init
+git remote add origin https://dev.pivoine.art/valknar/home.git
+git fetch && git checkout -f -t origin/main
+git config status.showUntrackedFiles no
+arty deps
 ```
 
 ---
@@ -98,9 +133,10 @@ arty docker:install         # Docker Engine with user group
 
 ### Custom Scripts
 
-Scripts in `~/scripts/` are loaded via PATH. They live in their own repo: `git@dev.pivoine.art:valknar/scripts.git`.
+Scripts live in `~/bin/` and are tracked in this repo.
 
-Available scripts: `stacks.sh` (Docker Compose stack manager), `arty.sh` (repository orchestration).
+- **`arty`** — repository orchestration, reads `arty.yml`
+- **`stacks`** — Docker Compose stack manager for `~/stacks`
 
 ---