[4b] Context Bundle assembly + cited MCP response

[andrmaz]

What to build

Replace the stub MCP response with real retrieval-grounded answers. For each authenticated MCP query, assemble a Context Bundle from the top-k ranked chunks (from #12), format a cited response payload referencing source documents, and return it through the /mcp entrypoint. Org isolation from #9 is enforced on every request.

Acceptance criteria

• MCP query path runs similarity search and assembles a Context Bundle.
• MCP response includes citations referencing the Document/Chunk sources used.
• Stub/mocked answers are fully retired.
• Retrieval is org-scoped on every request (no cross-org leakage).
• End-to-end integration test: upload doc → index → query → cited response.

Blocked by

• [4a] Org-scoped pgvector similarity search #12 [4a] Org-scoped pgvector similarity search
• [2c] Org isolation enforcement #9 [2c] Org isolation enforcement