Docs: Add GEMINI.md for project conventions (#352)

GEMINI.md

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+## Building and running
+
+Every time we make a set of changes you should run the following commands:
+
+Build the repo:
+npm run build
+
+Run tests:
+npm run test
+
+Type Check:
+npm run typecheck
+
+Lint and final checks:
+npm run preflight
+
+## Writing Tests
+
+When writing tests look at existing tests in packages/server and packages/cli to conform with the conventions in those files.
+
+## Git Repo
+
+The main branch for this project is called "main"
+
+## React
+
+Write idiomatic React code avoiding using useEffect and useRef when possible.
+
+## Comments policy
+
+Only write high-value comments if at all. Avoid talking to the user through comments.