feat: configured knowledge sources + hybrid retrieval

[manthan787]

Summary

• add createConfiguredRetriever to load .runbook/config.yaml|yml knowledge source configuration at runtime
• wire retriever creation in CLI/chat/Slack/MCP/integrations/evals to use configured sources instead of hardcoded filesystem-only defaults
• enable hybrid retrieval with reciprocal-rank fusion between FTS and vector results (when embeddings are configured)
• index chunk embeddings during sync and degrade gracefully to FTS-only if vector indexing/search fails
• normalize configured Notion/GitHub/API source auth with env fallbacks so configured sources are not silently skipped
• add retriever tests for config-driven source loading, fallback behavior, and knowledge.retrieval.topK defaults

Validation

• npm run test -- src/knowledge/retriever/__tests__/index.test.ts
• npm run test -- src/integrations/__tests__/hook-handlers.test.ts
• npm run test -- src/mcp/__tests__/server.test.ts
• npm run test -- src/slack/__tests__/gateway.test.ts
• npm run typecheck