Add a GEMINI.md file and .gitignore entry (#9279)

Author: kenzieschmoll

.gitignore

@@ -39,6 +39,9 @@ DEPENDENCIES.md
 **/logs/**
 *.stager_app.g.dart
 
+# Ignore Gemini logs
+**/.gemini/
+
 # Ignore VS code workspaces that may be configured per-user
 *.code-workspace
 

GEMINI.md

@@ -0,0 +1,24 @@
+# Repository rules
+
+When making changes to this code base, follow these rules, which are listed
+in no particular order:
+- `packages/devtools_shared` should never introduce a Flutter dependency or a
+dependency on web-only Dart libraries.
+- never import the `packages/devtools_app/lib/devtools_app.dart` file in code
+that lives under `packages/devtools_app/lib/src/`. This file is okay to import
+in code that lives under `packages/devtools_app/test/`.
+
+# Running tests
+
+Unit test and widget tests are all contained under a package's `test/`
+directory. These tests should be run with `flutter test` in all packages except
+for `packages/devtools_shared`, whose tests should be run with `dart test`.
+
+# Gemini Guidelines
+
+- Prefer to use MCP server tools over shell commands whenever possible.
+- When you are done making code changes, ensure the code does not have analysis
+errors or warnings. Also ensure it is formatted properly. You should have MCP
+server tools available to you to perform these tasks. If not, you can get
+analysis errors and warnings by running the `dart analyze` shell command, and
+you can perform Dart formatting with the `dart format` shell command.