docs: add CONTRIBUTING.md

CONTRIBUTING.md

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+# Contributing
+
+Potok is in active development. PRs are welcome — keep them small and tested.
+
+## Build and test
+
+You need Go 1.25+ (see `go.mod`).
+
+From the repo root:
+
+```bash
+go build ./...
+go test ./...
+```
+
+Those are the same commands CI runs (`.github/workflows/ci.yml`).
+
+To produce the two binaries:
+
+```bash
+go build -o potok ./cmd/potok     # CLI
+go build -o potokd ./cmd/potokd   # server
+```
+
+`potokd` reads `ADDR`, `DATA_DIR`, and `DATABASE_URL` from the environment (or a `.env` file in the working directory). There is no Makefile.
+
+Layout:
+
+| Path | What |
+|---|---|
+| `cmd/potok` | CLI |
+| `cmd/potokd` | Server |
+| `internal/` | Client, server, and protocol code |
+| `migrations/` | SQL migrations (embedded) |
+
+## Code style
+
+- `gofmt` everything before you push (`gofmt -w .`).
+- Match the file you're in. This is ordinary Go: cobra commands under `internal/client/cli`, packages under `internal/`, wrapped errors (`fmt.Errorf("...: %w", err)`).
+- Tests live next to the code (`*_test.go`) and use the standard `testing` package — no extra assertion libraries. Table-driven tests and `t.Helper()` are the usual pattern.
+- Don't add a linter, Makefile, or test framework unless the change needs it.
+
+## Pull requests
+
+1. Fork the repo.
+2. Create a branch off `main`.
+3. Make the change. Add or update tests if behaviour changes.
+4. Run `gofmt -w .` and `go test ./...`.
+5. Open a PR against `main`.
+
+Keep the PR focused. Say what changed and why. If it fixes an issue, mention it (`Fixes #58`).