docs: update README installation section

- Update prerequisites note to mention zsh
- Add step 3 (zsh install + chsh) and step 7 (exec zsh -l)
- Remove exec zsh from after-installation steps (script does it)

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

README.md

@@ -54,7 +54,7 @@
 curl https://dev.pivoine.art/valknar/home/raw/branch/main/bin/setup -sSf | sh
 ```
 
-**Prerequisites:** `curl` or `wget` · `sh`/`bash` · `sudo` *(only needed on Debian/Ubuntu if git is missing)*
+**Prerequisites:** `curl` or `wget` · `sh`/`bash` · `sudo` *(for installing git/zsh if missing)*
 
 ### What the setup script does
 
@@ -62,16 +62,15 @@ curl https://dev.pivoine.art/valknar/home/raw/branch/main/bin/setup -sSf | sh
 |------|--------|
 | 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 |
+| 3 | Installs `zsh` if missing, sets it as the default shell via `chsh` |
+| 4 | Clones this repo into `~` via HTTPS — or `git pull` if already present *(idempotent)* |
+| 5 | Runs `arty deps` — clones nvm, rbenv, pyenv, oh-my-zsh, powerlevel10k, zsh plugins |
+| 6 | Installs `arty` zsh completion via `arty completion zsh --install` |
+| 7 | Drops into a zsh login shell |
 
 ### 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