ds4-agent: separate runtime asset root from workspace cwd

`--chdir` (#221 / #232) lets `ds4-agent` find cwd-relative runtime assets such as `ds4flash.gguf` and `metal/*.metal`, but it also changes the agent's workspace.

This breaks the common launcher flow:

```sh
$ cd ~/workspaces/project
$ ds4-agent

```

In this case the agent should edit `~/workspaces/project`, while loading its model and Metal kernels from the ds4 root folder, eg`~/dev/ds4`.

My workaround is to keep cwd unchanged and pass absolute paths:

- `-m ~/dev/ds4/ds4flash.gguf`
- `DS4_METAL_*_SOURCE=~/dev/ds4/metal/*.metal`

That works, but it is bulky and easy to drift as kernel files change.

Request: add a separate option, for example:

```sh
ds4-agent --asset-root ~/dev/ds4
```

or:

```sh
ds4-agent --runtime-root ~/dev/ds4
```

This should make default runtime paths resolve from that root without changing process cwd / agent workspace.

`--chdir` can remain useful for server/service-manager cases, but `ds4-agent` needs a separate asset-root concept for local coding-agent workflows.