Add initial set of instructions for coding agents

AGENTS.md

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+# Project Instructions
+
+## After code editing
+* When you add, remove or modify files under pkg/formatter/templates/text/, run `make bindata` to regenerate pkg/formatter/templates/bindata.go. Commit updated bindata.go together with the template changes.
+* When you add, remove of modify commands, run `make docs` to regenerate the files under docs/. Commit updated docs together with the command changes.
+* Run `make format` and `make lint` to verify your code style. If these result in changes, evaluate if the changes are sensible. If they are sensible, commit them together with your code changes.
+* Run `make unit` to run the unit tests to verify your code changes.