Add remote write support

AGENTS.md

@@ -0,0 +1,5 @@
+# AGENTS.md
+
+## Rules
+
+- No unwraps allowed

CLAUDE.md

@@ -13,8 +13,22 @@ nix run              # Run directly
 cargo watch -x run   # Hot-reload during development (inside dev shell)
 ```
 
+## Testing
+
+```bash
+cargo test                                            # Unit tests only
+cargo test --test remote_write -- --ignored           # Integration tests (requires Docker/Podman)
+cargo test --test remote_write -- --ignored --nocapture # Integration tests with output
+```
+
+Integration tests spin up VictoriaMetrics and Prometheus containers via testcontainers, push metrics through the real `RemoteWriteSender::push_once` code path, and query back to verify correctness. Each test suite runs against both backends.
+
 ## Flake Maintenance
 
 When `Cargo.toml` changes (version bump or dependency changes):
 - Update `version` in `flake.nix` to match `Cargo.toml`
 - Set `cargoHash` to `""` and rebuild to get the new hash from the error output, then update it
+
+## Rules
+
+- No unwraps allowed