Add post-install tips for Stripe Projects plugin

[rami-stripe]

Summary

• Expand the projects plugin post-install message from a single URL to actionable next steps
• Shows key commands (catalog, init, --help), docs/site links, feedback email, and the AI coding agent skill install command
• Adds test coverage for the new projects tips

Output after install

✔ installation complete.

  stripe projects catalog    Browse available services
  stripe projects init       Set up a new project
  stripe projects --help     See all commands

Docs:     https://docs.stripe.com/projects
Site:     https://projects.dev
Feedback: projects-feedback@stripe.com

For AI coding agents:
  npx skills add https://github.com/stripe/ai --skill stripe-projects -y -g

Test plan

• go test ./pkg/cmd/plugin/postinstall/ passes
• Install projects plugin and verify tips print after install
• Upgrade projects plugin and verify tips print after upgrade

🤖 Generated with Claude Code

[svenugopal-stripe]

r? @tomer-stripe