docs: Add README for Agent_Step18_CompactionPipeline

[Copilot]

No developer documentation existed for the Agent_Step18_CompactionPipeline sample, leaving the compaction API undiscovered and unexplained.

Changes

• New README.md at dotnet/samples/02-agents/Agents/Agent_Step18_CompactionPipeline/
  • Explains why context compaction is needed and how the framework models it (MessageIndex, atomic groups, trigger/target lifecycle)
  • Reference table of all five built-in strategies (ToolResultCompactionStrategy, SummarizationCompactionStrategy, SlidingWindowCompactionStrategy, TruncationCompactionStrategy, ChatReducerCompactionStrategy) with aggressiveness level and use-case guidance
  • Reference table of all CompactionTriggers factory methods plus All/Any/Always/Never
  • Prerequisites including the [Experimental]/AGENTSAI001 suppression note and the two-client pattern for the summarizer
  • Environment variable setup, run instructions, and annotated expected output describing which strategy fires at each threshold
  • Key-concept snippets: single-strategy, IChatReducer adapter, custom trigger lambda, combined triggers

Contribution Checklist

• The code builds clean without any errors or warnings
• The PR follows the Contribution Guidelines
• All unit tests pass, and I have added new tests where possible
• Is this a breaking change? If yes, add "[BREAKING]" prefix to the title of the PR.

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