Keep shared skill caches fresh across plugin loads

[copyberry]

Keep shared skill caches fresh across plugin loads

What changed

• Key cached skill snapshots by filesystem and plugin snapshot identity so compatible config and working-directory loads can share results without reusing stale plugin data.
• Coalesce concurrent loads for the same cache key and replace the cached entry on forced reload.
• Clear both plugin and skill caches when skills/list forces a reload, and bypass working-directory caching when effective plugin roots are present.

Testing

• Cover concurrent cache sharing, filesystem isolation, forced reloads, and refreshed plugin skill metadata in host service and app server tests.

[github-actions]

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