[P2-Enhancement] Add explain API for search result debugging

[VirtualAgentics]

Description

Add explain API endpoint to help developers understand and debug search results by showing the factors that contributed to each result's ranking.

Background

This enhancement extends Issue #58 (hybrid search) by providing debugging capabilities that help developers understand why certain results were returned and how to tune search parameters.

Requirements

• Add /v1/search/explain endpoint
• Show scoring factors: similarity, BM25, freshness, quality
• Display individual factor contributions
• Add debugging information for search tuning
• Include result metadata and processing details
• Add explain API documentation
• Add explain API tests

Implementation Details

Files to modify:

• src/contextforge_memory/main.py - Add explain endpoint
• src/contextforge_memory/search/ - Add explain functionality
• src/contextforge_memory/explain/ - New explain module
• tests/ - Add explain tests
• README.md - Document explain API

Technical approach:

• Create explain service that breaks down search scoring
• Add detailed result analysis with factor contributions
• Implement debugging information collection
• Add explain response formatting
• Create explain API documentation

Acceptance Criteria

• Explain endpoint returns detailed scoring breakdown
• All scoring factors are explained clearly
• Debugging information is helpful for tuning
• API documentation is complete
• Tests cover explain functionality

Testing Requirements

• Unit tests for explain logic
• Integration tests for explain endpoint
• Performance tests for explain overhead
• Documentation tests for explain API

Documentation Updates

• README.md - Document explain API
• API documentation - Explain endpoint
• Debugging guide - Search tuning
• Examples - Explain API usage

Related Issues

• Extends: [P2] Add hybrid search with BM25 + semantic vectors #58 (hybrid search)
• Depends on: None
• Blocks: None