agent: resolve runtime assets from binary directory

[dbader]

Adds ds4-agent runtime-root detection while keeping the launch cwd as the agent workspace, fixing #240

Details:

• ds4-agent now detects its executable directory as the default runtime root.
• --runtime-root remains available as an override.
• DS4_RUNTIME_ROOT is passed through for Metal source lookup.
• Metal kernels can resolve from the runtime root without per-kernel env vars.
• README documents the new launcher behavior.

Follow-up to #221/#232: --chdir changes the workspace cwd, but the agent use case needs asset lookup separate from cwd.