chore: replace ancient tunnel + proxy-from-env with proxy-agent

[sorccu]

⚠️ WARNING: This PR currently has an issue where the proxy server port gets attached to port-lacking destination URLs. So, https://api.checklyhq.com becomes https://api.checklyhq.com:8080 if https_proxy=http://localhost:8080, for example. The PR cannot be merged until this issue is resolved.

The tunnel package we use has not been updated for 7 years and it's a bit unclear if it still does what it's supposed to in recent Node versions. I was having some trouble getting MITM to work today and rather than diving deep into an ancient package, I figured we should update to something more modern.

This change is better for security, and potentially enables new types of proxy-reliant setups with no effort from our side.

Affected Components

• CLI
• Create CLI
• Test
• Docs
• Examples
• Other

Notes for the Reviewer

Resolves #[issue-number]

New Dependency Submission

[github-actions]

⚠️ AI context is out of date

The skills/ directory doesn't match the generated output from packages/cli.

Please run the following locally and commit the changes:

npm run prepare