sandboxer: refactor Layout from struct to interface

[dungdm93]

Overview

Replace sandboxer.Layout from a plain struct with fields to a Go interface with methods. This decouples consumers from any specific layout implementation and makes it straightforward to supply custom layouts (e.g. in tests or future sandbox backends).

Design

Before: Layout was a struct with public string fields (Workspace, Temp, Actions, Tools, Runtimes). All sandboxes returned *Layout.

After: Layout is an interface exposing those five paths as methods. A concrete StandardLayout(base string) type implements it by joining the base directory with each well-known subdirectory name. Sandboxes return Layout (interface value).

type Layout interface {
    Workspace() string
    Temp()      string
    Actions()   string
    Tools()     string
    Runtimes()  string
}

type StandardLayout string   // implements Layout via path.Join

container.DefaultLayout becomes a package-level StandardLayout value rather than a factory function, eliminating the DefaultLayout("") call pattern.

Changes

• sandboxer/sandbox.go — define Layout interface and StandardLayout concrete type
• host, incus, container sandbox implementations — adopt StandardLayout, return Layout interface
• All consumers (executor, command, runtime, wire) — update to call interface methods
• mock/sandboxer/sandbox.go — regenerated; adds MockLayout for test use
• Tests — updated to use MockLayout (custom paths) or StandardLayout (derived paths) instead of struct literals