feat: add native Linux mode (vmType=native) — bypass Lima/QEMU on Linux hosts

[architectureman]

Motivation

On Linux hosts, Colima currently creates a full Linux VM via Lima/QEMU even though the host itself is already running Linux. This adds unnecessary overhead in terms of CPU, memory, disk, and startup time. Many Linux users expect container runtimes to run natively — the same way Docker Desktop works on Linux.

This PR introduces vmType: "native" which allows Colima on Linux to use the host's container runtime directly, eliminating the virtualization layer entirely while keeping the same Colima UX (profiles, colima start/stop/status/delete, Docker context management, Kubernetes provisioning, etc.).

What's New

# On Linux — auto-detected, zero config needed
colima start                         # uses native mode by default

# Or explicitly
colima start --vm-type native        # bypass VM, use host Docker/containerd/Incus
colima start --vm-type qemu          # traditional VM mode (still fully supported)

How It Works

A new environment/vm/native/ package implements the environment.VM interface using direct host execution (os/exec) instead of SSH-based Lima commands:

Component	VM Mode (existing)	Native Mode (new)
Command execution	limactl shell ... (SSH)	Direct os/exec.Command
File I/O	SFTP via Lima	os.ReadFile/os.WriteFile
Docker socket	~/.colima/.../docker.sock (forwarded)	/var/run/docker.sock
Resource info	limautil.Instance()	/proc/cpuinfo + syscall.Statfs
Host gateway	host.lima.internal	ip route show default
Network	Lima virtual network	Host network (direct)

Safety Guarantees

• Zero impact on macOS — native mode is Linux-only; all existing VM paths remain untouched
• Opt-out available — users can still use --vm-type qemu on Linux if they prefer isolation
• Guard pattern — all limautil.* calls are guarded with conf.VMType != "native" checks
• colima model — intentionally blocked in native mode for the initial release (GPU passthrough setup differs from krunkit)

Changes Summary

New files (7):

• util/linux.go — Linux() detection helper
• environment/vm/native/{native,shell,file,config,hostinfo}.go — Full VM interface implementation
• model/guest.go — Centralized newGuest() factory (replaces 13× hardcoded lima.New())

Modified files (18):

• environment/vm.go — DefaultVMType() returns native on Linux
• config/config.go — DriverLabel() returns "Native"
• config/configmanager/configmanager.go — Skip VM validations for native
• app/app.go — NewWithVMType() factory + limautil guards
• cmd/{start,restart,clone,ssh-config,prune,util}.go — CLI integration
• environment/container/docker/{daemon,context}.go — Socket + gateway fallback
• environment/container/kubernetes/{k3s,kubeconfig}.go — IP address guards
• environment/container/incus/incus.go — Disk provisioning guard
• model/{docker,ramalama,runner}.go — newGuest() migration

Testing

Test	Result
Cross-compile GOOS=linux GOARCH=amd64	✅
macOS regression GOOS=darwin GOARCH=arm64	✅
go vet ./...	✅
E2E: start → status → stop → delete (in Colima VM)	✅

E2E Test Output (click to expand)

$ colima-test start --vm-type native -p native-test
INFO[0000] starting colima [profile=native-test]
INFO[0000] runtime: docker
INFO[0000] starting (native mode) ...     context=vm
INFO[0000] provisioning ...               context=docker
> Successfully created context "colima-native-test"
> Current context is now "colima-native-test"
INFO[0014] starting ...                   context=docker
INFO[0014] done

$ colima-test status -p native-test
INFO[0000] colima [profile=native-test] is running using Native
INFO[0000] arch: x86_64
INFO[0000] runtime: docker
INFO[0000] docker socket: unix:///var/run/docker.sock

$ colima-test status -p native-test --json
{"driver":"Native","cpu":2,"memory":2061250560,"disk":19682557952,...}

$ colima-test stop -p native-test
INFO[0000] stopping colima [profile=native-test]
INFO[0010] done

$ colima-test delete -p native-test --force
INFO[0000] deleting colima [profile=native-test]
INFO[0000] done

Use Cases

1. CI/CD pipelines — Run Colima in GitHub Actions/GitLab CI Linux runners without QEMU overhead
2. Linux workstations — Developers on Linux get zero-overhead container management with Colima's UX
3. Server deployments — Unified Colima interface across macOS (VM) and Linux (native) environments
4. GUI management — Community projects like Colima UI (a Tauri-based desktop app providing visual management for Colima instances) already support this native mode, showing growing ecosystem adoption and enabling Linux users to manage Docker/K8s visually without any VM overhead

Dependencies

• Linux host with systemd
• Container runtime pre-installed (Docker, containerd, or Incus)
• User in the docker group (for Docker runtime)

Future Work

• colima model support in native mode (NVIDIA Container Toolkit integration)
• Native mode for WSL2 (Windows Subsystem for Linux)
• Unit tests for environment/vm/native/ package