.NET: Consolidate skill-source caching and make skill sources disposable #6830

Description

Skill caching is duplicated - an inline cache in AgentMcpSkillsSource and the generic CachingAgentSkillsSource decorator - and the decorator masks the inline one when the builder wraps the MCP source. Skill sources also hold disposable resources with no way to release them, and ArchiveEntryLoader reconciles a shared on-disk directory on every call with no protection against concurrent corruption.

Proposed Solution

• Make CachingAgentSkillsSource the single cache layer; remove the inline cache from AgentMcpSkillsSource.
• Make AgentSkillsSource (and its decorators) IDisposable, with an ownsSource flag on AgentSkillsProvider so it disposes the source only when it owns it.
• Guard ArchiveEntryLoader's reconcile/extract/read with a per-instance lock.

The affected types are [Experimental].