[WIP] Add chat-history compaction feature for agent framework

[Copilot]

Thanks for asking me to work on this. I will get started on it and keep this PR's description up to date as I form a plan and make progress.

Write me a blog post in markdown that describe the chat-history compaction feature for the microsoft agent framework.

agent framework repo: https://github.com/microsoft/agent-framework

compaction feature branch: copilot/create-message-compaction-provider

compaction feature source: dotnet\src\Microsoft.Agents.AI\Compaction

compaction feature sample: dotnet\samples\02-agents\Agents\Agent_Step18_CompactionPipeline

Explain that compaction occurs for all model invocations, including the function calling loop.

Show pattern for creating an agent with CompactionProvider

Describe various compaction strategies highlighting usage of CompactionTriggers

Note, this is just one section of a larger blog...so be concise without sacrificing information.

Create the blog post as docs/compaction.md

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