feat(scripts): add one-line install script

- Add `scripts/install.sh` — a single `curl | bash` installer that handles system deps, Rust, clone, build, and install
  automatically.
- Update README Linux/macOS section with a "One-Line Installer (Recommended)" block above the existing manual steps.

1. Detects OS (Linux apt/dnf, macOS Xcode CLT)
2. Installs build deps + git via system package manager (sudo only here)
3. Installs Rust via rustup (skipped if already present)
4. Shallow-clones the repo to `/tmp/zeroclaw-install`
5. `cargo build --release --locked` + `cargo install --path . --force --locked`
6. Cleans up temp dir and prints next steps (`source ~/.cargo/env`, `zeroclaw onboard`)

README.md

@@ -92,6 +92,14 @@ ls -lh target/release/zeroclaw
    cargo --version
    ```
 
+#### One-Line Installer
+
+Or skip the steps above and install everything (system deps, Rust, ZeroClaw) in a single command:
+
+```bash
+curl -LsSf https://raw.githubusercontent.com/zeroclaw-labs/zeroclaw/main/scripts/install.sh | bash
+```
+
 #### Optional
 
 - **Docker Desktop** — required only if using the [Docker sandboxed runtime](#runtime-support-current) (`runtime.kind = "docker"`). Install via `winget install Docker.DockerDesktop`.