Add context compression feature for long conversations

- Implemented automatic context compression to manage long conversations that approach the model's context limit.
- Configured the feature to summarize middle turns while protecting the first three and last four turns, ensuring important context is retained.
- Added configuration options in `cli-config.yaml` and environment variables for enabling/disabling compression and setting thresholds.
- Updated documentation in `README.md`, `cli.md`, and `.env.example` to explain the context compression functionality and its configuration.
- Enhanced the `cli.py` to load compression settings into environment variables, ensuring seamless integration with the CLI.
- Completed the implementation of context compression as outlined in the TODO list, marking it as a significant enhancement to conversation management.

.env.example

@@ -154,3 +154,13 @@ WEB_TOOLS_DEBUG=false
 VISION_TOOLS_DEBUG=false
 MOA_TOOLS_DEBUG=false
 IMAGE_TOOLS_DEBUG=false
+
+# =============================================================================
+# CONTEXT COMPRESSION (Auto-shrinks long conversations)
+# =============================================================================
+# When conversation approaches model's context limit, middle turns are
+# automatically summarized to free up space.
+#
+# CONTEXT_COMPRESSION_ENABLED=true        # Enable auto-compression (default: true)
+# CONTEXT_COMPRESSION_THRESHOLD=0.85      # Compress at 85% of context limit
+# CONTEXT_COMPRESSION_MODEL=google/gemini-2.0-flash-001  # Fast model for summaries