Honor layered configuration when loading plugins (#40954)

## What changed

- Resolve plugin activation, MCP server policy, and marketplace definitions from the effective configuration stack, including system settings and trusted project overrides.
- Load plugin skills independently for each working directory passed to `skills/list`.
- Exclude project configuration from catalog requests whose `cwds` are omitted or empty.
- Cache up to eight loaded plugin configurations with least-recently-used eviction so requests for different working directories can reuse their results.

## Testing

- Cover layered plugin and MCP policy across trusted and untrusted projects.
- Cover per-directory skill and plugin catalog results, cache reuse and eviction, and system-defined marketplaces.

GitOrigin-RevId: cb68b2b1bdab1da48e05ca7562bc05014431fc71