feat(retrieval): add search diagnostics

[plind-junior]

Summary

• add vouch search-diagnose for explaining why a target artifact does or does not appear in retrieval
• expose the same diagnostic through kb.search_diagnose on MCP and JSONL
• report target existence, derived-index presence, raw/scoped rank, backend, viewer scope, warnings, and returned hits
• add focused tests and advertise the new method in capabilities

Tests

• VOUCH_USER=human-reviewer .venv/bin/python -m pytest tests/ -q --ignore=tests/embeddings
• .venv/bin/python -m mypy src
• .venv/bin/python -m ruff check src tests

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🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch feat/search-diagnostics

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