docs: add agent contribution rules

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

AGENT.md

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+# Project notes for agents
+
+## AGENT RULES
+- you **MUST** never do `git push`, unless you're explicitly told to
+- you **MUST** always run formatting tools such as `cargo fmt` after you changed the contents of a file
+- you **MUST** use conventional commits when committing (`feat`, `fix`, `perf`, `refactor`, `chore`, `docs`), with `!` and a `BREAKING CHANGE:` footer for API breaks
+- you **MUST** end commit messages with the trailer `Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>`
+- you **MUST** inform the user about commits or any other changes to git, including the commit message
+- you **SHOULD** commit your changes after every step, but only when `cargo clippy` and `cargo test` pass
+- you **SHOULD** commit formatting-only changes separately from functional changes
+
+## Commands
+
+```sh
+cargo build --workspace --all-targets
+cargo test --workspace
+cargo clippy --workspace --all-targets -- -D warnings   # must stay clean
+cargo fmt --all
+```
+
+## Conventions
+
+- Comment only code that needs clarification.
+- Warnings are treated as errors (`-D warnings`).