ds4-agent: separate runtime asset root from workspace cwd

[dbader]

--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:

$ cd ~/workspaces/project
$ ds4-agent
<have agent work in ~/workspaces/project>

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:

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

or:

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.

[dbader]

And we should probably make ds4-agent infer a runtime root from its own executable path when none is provided, to just have that common use case work out of the box?