One-shot install script (Node ≥22.19 + dsh) #84

Why

Install friction is the main growth bottleneck after the product/demo story (from open-source benchmarking). Newcomers should not have to manually wire Node + dsh before first run.

Ask

Ship a one-shot install script that:

1. Detects / requires Node ≥ 22.19 (or installs via a documented path)
2. Ensures dsh is available (install or clear error)
3. Installs codsh (npm global or documented alternative)
4. Prints next step (API key / settings) on success

Non-goals

• Replacing WSL guidance for Windows (Native Windows (pwsh): what actually works #62 covers native Windows feedback)
• Brew / multi-channel packaging (tracked separately)

Acceptance

• Documented one-liner in README EN/ZH
• Script fails loudly with actionable message when Node/dsh missing
• Happy path reaches a runnable codsh --version

Deduped: not covered by #62/#63/#64/#76–#83.