Add agent template for Jerboa app repos

ober

2b05d0b58f6e185002f7d260327de4529525777a

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+# AGENT.md for jerboa-$app Repositories
+
+These instructions apply to agents working in this repository.
+
+## Build Contract
+
+The Makefile must provide these targets:
+
+- `make` prints the available targets. It must not build, install, test, or mutate artifacts.
+- `make binary` builds the application binary for the local host OS and architecture.
+- `make linux-amd64` cross-builds the Linux amd64 binary from the current host.
+- `make linux-arm64` cross-builds the Linux arm64 binary from the current host.
+- `make freebsd-amd64` cross-builds the FreeBSD amd64 binary from the current host.
+
+The cross-build targets must use the local cross toolchains and Jerboa/Chez
+cross-build setup. Never use Docker or Podman for building, testing, smoke
+testing, packaging, or verifying these targets.
+
+## Required Verification
+
+Before committing or pushing any code, all required build targets must succeed:
+
+```sh
+make
+make binary
+make linux-amd64
+make linux-arm64
+make freebsd-amd64
+```
+
+Do not commit or push if any target fails. If a cross-build cannot run because
+the host is missing a required toolchain or sysroot, stop and report the missing
+dependency instead of committing or pushing partially verified code.
+
+## Working Rules
+
+- Keep changes inside the current repository unless the user explicitly names another path.
+- Preserve the target semantics above when editing the Makefile.
+- Treat the cross-build targets as release-critical, not optional follow-up work.
+- Do not add Dockerfiles, container scripts, Podman invocations, or container-based verification paths.