Support for Goose agent

[EliRibble]

goose is currently my agent of choice, so I added support for it.

[beyond5959]

Thanks for adding Goose support. The adapter follows the existing ACP CLI pattern nicely, but I’d like one security/protocol-boundary change before this lands.

internal/agents/goose/goose.go currently initializes Goose with only clientInfo and protocolVersion. For a new third-party ACP provider, we should explicitly advertise fail-closed client capabilities, consistent with the existing ACP stdio providers and the repo spec:

"clientCapabilities": map[string]any{
    "fs": map[string]any{
        "readTextFile":  false,
        "writeTextFile": false,
    },
    "terminal": false,
},

The current transport will return method-not-found for unsupported inbound requests, so I don’t think this is an immediate file-read/write authorization bug. Still, making the capability boundary explicit avoids Goose or a future ACP implementation inferring capabilities we do not intend to provide.

Please also add a small fake-process test that asserts the initialize params include those disabled capabilities. After that, this should be in good shape.