feat: URL-based import (Gist/GitHub)

[HerbHall]

Summary

Accept a URL in the import dialog to pull runbook collections from GitHub Gists or raw JSON URLs.

Research Context

• Navi: navi repo add <url> imports community cheatsheets from GitHub repos
• Portainer: Templates from Git repos (URL + branch + path)
• Yacht: Decentralized template marketplace via URL-based template collections

Proposed UX

• Import dialog: add a "From URL" tab alongside the current paste method
• Accepts: GitHub Gist URL, raw JSON URL, GitHub repo file URL
• Validates JSON structure before importing
• Shows preview of runbooks to be imported with confirm/cancel

Sharing Workflow

• User exports runbooks as JSON
• Pastes JSON into a GitHub Gist
• Shares Gist URL with team
• Recipients import via URL in one click