Revise coaiapy-mcp to use library imports instead of subprocess

Updated IMPLEMENTATION_PLAN.md to specify library import approach
instead of subprocess CLI wrappers. This avoids environment variable
propagation issues and provides cleaner, faster implementation.

Key changes:
- Architecture: Direct imports from coaiapy, langfuse, redis libraries
- Configuration: Load from coaiapy.coaiamodule.load_config() once
- Tools: Call library functions directly (no subprocess.run())
- Benefits: Faster, cleaner error handling, no env var issues

Closes #44

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Co-Authored-By: Claude <noreply@anthropic.com>

Author: jgwill