Scope runtime workspace roots to execution environments

[copyberry]

Scope runtime workspace roots to execution environments

Why

Multi-environment turns can run against different filesystems, so a single
thread-wide workspace-root list cannot accurately describe every selected
environment.

What changed

• Add optional runtimeWorkspaceRoots to each app-server environment selection.
  Omitted roots default to that environment's cwd, while an empty list selects
  no roots.
• Carry environment-native roots through turn selection, model-visible context,
  permission materialization, and tool sandboxing.
• Keep the top-level runtimeWorkspaceRoots field as a compatibility input when
  constructing default environment selections; explicit selections use their
  own roots.

Testing

• Cover thread and turn API defaults, overrides, empty roots, and schema
  round-tripping.
• Verify foreign roots remain model-visible and remote execution applies the
  target environment's roots before sandbox selection.