Add CONTRIBUTING.md with test and release instructions

CONTRIBUTING.md

@@ -0,0 +1,48 @@
+# Contributing
+
+## Prerequisites
+
+- Go 1.23+
+- [pre-commit](https://pre-commit.com/) (optional, but recommended)
+
+## Setup
+
+```bash
+go mod tidy
+make install-hooks   # sets up pre-commit (gofmt, go vet, golangci-lint)
+```
+
+## Running Tests
+
+```bash
+make test              # all tests
+make unit-test         # unit tests only (-short -race)
+make integration-test  # integration tests only (-race)
+```
+
+## Other Checks
+
+```bash
+make lint        # go vet
+make fmt-check   # verify gofmt formatting
+make build       # build binary to bin/paude-proxy
+make docker      # build container image with podman
+```
+
+## Cutting a Release
+
+Releases are fully automated via GitHub Actions. Push a semver tag to trigger the pipeline:
+
+```bash
+git tag v1.2.3
+git push origin v1.2.3
+```
+
+The [release workflow](.github/workflows/release.yml) will:
+
+1. Lint and run all tests
+2. Build binaries for linux/amd64, linux/arm64, darwin/amd64, darwin/arm64
+3. Build and push multi-arch container images to `quay.io/bbrowning/paude-proxy`
+4. Create a GitHub Release with auto-generated notes and the binaries attached
+
+**Stable vs pre-release:** Tags matching `vX.Y.Z` exactly (e.g. `v1.2.3`) are treated as stable releases and update the `latest` container tag. Anything else (e.g. `v1.2.3-rc1`) is marked as a pre-release and does not update `latest`.