Agents source control in Github

[avaer]

We should make it easy for developers to source control their agents and track changes made to them.

• Add .gitignore to created agents so it is clear which files should/should not be to git
• Document at https://docs.upstreet.ai/ how developers can push the agent to a remote repository\
• Document how agent edits/updates can be pushed

Note: this issue is about managing the agent itself in git source control. This issue is not about API access to Github PRs -- that is a separate issue: https://github.com/UpstreetAI/monorepo/issues/513