[Ubuntu 24.04][Install] installer fails without Docker running, so Podman-only machines cannot install

## Description

The NemoClaw installer stops with an error when the Docker daemon is not running, so NemoClaw cannot be installed on a host that has only Podman.

Before failing, the installer also tries to enable and start `docker.service` on the user's behalf.

This conflicts with the documented deployment options. Native rootless Podman is listed as a supported deployment path, selected with `NEMOCLAW_GATEWAY_RUNTIME=podman`, but that path cannot be reached on a Podman-only host because installation fails first. In practice Docker must be installed and running before the Podman path can be chosen at all.

- **Platform scope:** Reproduced on Ubuntu 24.04 x86_64 only; other platforms not tested.
- **Regression:** Unknown — earlier versions not tested for this path.
- **OpenShell issue:** No

## Environment

```text
Device: Ubuntu 24.04 host, x86_64, no GPU
OS: Ubuntu 24.04.4 LTS
Architecture: x86_64
Node.js: Not on PATH (installer-managed Node.js)
npm: Not on PATH (installer-managed Node.js)
Docker: Docker version 29.8.0, build 88096ef (installed, daemon stopped for this test)
Podman: podman version 5.7.0 (rootless, user socket active)
OpenShell CLI: N/A (install failed)
NemoClaw: N/A (install failed)
OpenClaw: N/A (install failed)
```

## Steps to Reproduce

1. Prepare a host with rootless Podman 5.7.0 installed and its user socket active. Confirm the Podman API answers:

```bash
podman info --format "{{.Host.RemoteSocket.Exists}}"
```

2. Make sure the Docker daemon is not running, to represent a host that does not use Docker:

```bash
sudo systemctl stop docker
systemctl is-active docker
```

3. Run the ordinary NemoClaw installer, pinned to the target version.

4. Read the installer output and its exit status.

## Expected Result

Installation succeeds on a host whose container runtime is rootless Podman, so that the documented native rootless Podman deployment path can then be selected.

If Docker really is required at install time regardless of the runtime chosen later, the installer says so plainly, and the documentation states that Docker is a prerequisite for every deployment path including the Podman one.

## Actual Result

The installer aborts:

```text
[INFO] The Docker daemon is not running.
[INFO] The next step uses sudo to enable and start the docker.service unit.
[WARN] Could not enable docker.service — will verify daemon accessibility below.
[ERROR] Docker is installed but not reachable. Try: sudo systemctl start docker
```

Installation does not complete, so no NemoClaw CLI is available and the documented Podman deployment path cannot be selected afterwards.

The installer also attempts to enable and start `docker.service` on the user's behalf rather than recognizing the rootless Podman runtime that is present and answering.

For contrast, the platform support documentation lists native rootless Podman as a supported deployment option selected with `NEMOCLAW_GATEWAY_RUNTIME=podman`, and does not state that a running Docker daemon is required before that option can be used.

## Logs

```text
Not captured beyond the terminal output quoted in Actual Result.
```

Start at the installer path that checks Docker availability and selects the gateway runtime; trace how NEMOCLAW_GATEWAY_RUNTIME=podman is handled before the Docker service is enabled or started. Reproduce on the Ubuntu 24.04 Podman-only setup described here, and confirm that installation reaches the documented Podman deployment path without requiring a running Docker daemon.

Written by the indexing model from the issue text.