Add npm install path (npx / npm i) alongside curl|bash installer

[ThinkOffApp]

From claudemm's repo review (2026-07-06). The README leads with curl -fsSL ... | bash, which is a trust/friction barrier for new users of our most-starred repo (★43).

Goal: offer a standard package-manager install alongside the one-shot script.

Scope:

• Publish to npm (or GitHub Packages) as @thinkoffapp/ide-agent-kit (or similar).
• Support npx ide-agent-kit init and npm i -g.
• Keep the curl|bash script for the full brew-prereqs macOS path, but document npm as the lighter option.
• Update README install section to lead with the lower-friction choice.