docs: add HACKING.md guide on setting up a local dev environment

HACKING.md

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+# Developing Uncloud
+
+A short guide on setting up your local development environment.
+
+## Prerequisites
+
+- [mise](https://mise.jdx.dev/) (development environment setup tool)
+- [Docker](https://docs.docker.com/get-docker/) (for running end-to-end tests)
+
+## Setup
+
+Uncloud uses `mise` to install and manage development tools and dependencies. It makes it easy to create reproducible
+development environments on developer machines and CI.
+
+Follow the [Installing Mise](https://mise.jdx.dev/installing-mise.html) guide to get it set up on your machine.
+
+Mark the project as trusted and install all required tools (Go, `protoc`, `golangci-lint`, etc.) by running from the
+project root:
+
+```shell
+mise trust
+mise install
+```
+
+This reads [`mise.toml`](mise.toml) and installs the exact versions of tools specified there. They will be available in
+your `PATH` when you are in the project directory, so you can run `go` or `golangci-lint` directly. The tools won't be
+available outside the project, so they don't interfere with system packages.
+
+## Building
+
+Build the CLI:
+
+```shell
+go build -o uc ./cmd/uncloud
+```
+
+Or build and run the CLI with a single command:
+
+```shell
+go run ./cmd/uncloud --help
+```
+
+The Uncloud daemon (`uncloudd`) only supports Linux, so you need to cross-compile it if you're developing on macOS or
+Windows:
+
+```shell
+GOOS=linux GOARCH=amd64 go build -o uncloudd ./cmd/uncloudd
+```
+
+## Testing
+
+Run all tests (unit and e2e):
+
+```shell
+make test
+```
+
+### End-to-end tests
+
+E2e tests run inside Docker using the [`ucind`](Dockerfile) (Uncloud-in-Docker) image. Build it first:
+
+```shell
+make ucind-image
+```
+
+⚠️ NOTE: You need to rebuild the `ucind` image every time you make changes to the daemon code if you want to test them
+in e2e tests.
+
+Then run the tests:
+
+```shell
+make test-e2e
+```
+
+If tests leave leftover containers, clean them up with:
+
+```shell
+make test-clean
+```
+
+## Linting and formatting
+
+Lint and format the code:
+
+```shell
+make lint-and-fix
+```
+
+## Code generation
+
+Update the generated Go code for the machine gRPC API after modifying `.proto` files:
+
+```shell
+mise run proto
+```
+
+Regenerate mocks:
+
+```shell
+make mocks
+```