Add README.md for Agent_Step18_CompactionPipeline

[Copilot]

Adds developer documentation for the Agent_Step18_CompactionPipeline sample, which was missing a README.

Changes

• New file: dotnet/samples/02-agents/Agents/Agent_Step18_CompactionPipeline/README.md
  • Explains the four-strategy compaction ladder (ToolResultCompactionStrategy → SummarizationCompactionStrategy → SlidingWindowCompactionStrategy → TruncationCompactionStrategy) with a reference table covering aggressiveness, trigger condition, and behavior for each
  • Shows the ChatHistoryCompactionPipeline.Create() factory shortcut for Aggressive, Balanced, and Gentle approaches
  • Shows how to wire the pipeline as ChatReducer on InMemoryChatHistoryProvider
  • Covers prerequisites, environment variables, run instructions, and expected output

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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