docs: add AGENTS.md for coding agents

AGENTS.md

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+# AGENTS.md
+
+Rust libraries for Prometheus and OpenMetrics scrape pipelines. See [README.md](README.md) for scope and layout.
+
+## Verify your change
+
+Run these before you push. CI runs the same commands, see [.github/workflows/ci.yml](.github/workflows/ci.yml).
+
+```sh
+cargo fmt --all -- --check
+cargo clippy --all-targets --all-features -- -D warnings
+cargo clippy --all-targets --no-default-features -- -D warnings
+cargo test --all-features
+cargo test --no-default-features
+```
+
+If you touch `src/arrow`, also run the OTAP compatibility suite. It has its own manifest:
+
+```sh
+cargo test --manifest-path tests/otap-compat/Cargo.toml
+```
+
+Do not use Rust language or std features newer than `rust-version` in `Cargo.toml`.
+
+## Spec
+
+[specs/expfmt/SPEC.md](specs/expfmt/SPEC.md) is the contract for `expfmt`. If you change behavior the spec describes, update the spec in the same PR.
+
+## Test data
+
+Do not edit files under `testdata/` by hand. They are generated from the Go modules pinned in `tests/go-harness/go.mod`. To regenerate them (requires Go):
+
+```sh
+cd tests/go-harness && go generate ./... && go test ./...
+```
+
+CI regenerates them and fails if `git diff --exit-code testdata` shows a change.
+
+## Go is the reference
+
+The Go code in `prometheus/prometheus` (`model/textparse`) and `prometheus/common` (`expfmt`) is the reference. Expected outputs in `testdata/` come from running the Go parsers. If Rust and Go disagree, fix the Rust code, not the fixture.
+
+If Rust must differ from Go on purpose, explain each difference in the PR body with a permalink to the Go code.
+
+## Commits and PRs
+
+- Sign off every commit with `git commit -s` (DCO).
+- PR titles use `<area>: <summary>`, for example `parse: add OpenMetrics 1.0 parser`.
+- Link the GitHub issue the PR closes.

CLAUDE.md

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+@AGENTS.md