feat(mcp): expose streamable http tools

[HugoSkylight]

Summary

OpenCortex can now be used by MCP clients over the current Streamable HTTP transport. The new /mcp endpoint exposes the cleaned write and recall chain as JSON-RPC tools without reintroducing the legacy HTTP+SSE transport.

The endpoint supports initialize, ping, tools/list, and tools/call. Tool calls are backed by the existing memory/resource/session/forget/search flows, so MCP requests share the same runtime dependencies, identity context, worker side effects, and retrieval behavior as the REST API.

Details

• Implements a non-streaming Streamable HTTP server shape: POST /mcp returns JSON-RPC JSON responses, while GET /mcp returns 405 because SSE sessions are not implemented.
• Validates MCP transport headers before resolving runtime dependencies, requiring Content-Type: application/json and Accept: application/json, text/event-stream.
• Exposes opencortex.search, opencortex.store_memory, opencortex.store_resource, opencortex.forget, opencortex.session_message, and opencortex.session_end with Pydantic-derived input schemas.
• Keeps MCP protocol models and tool dispatch isolated under src/opencortex/mcp/ so the REST routes stay focused on HTTP API concerns.

Validation

• uv run --group dev ruff format --check src/opencortex tests/opencortex
• uv run --group dev ruff check src/opencortex tests/opencortex
• uv run --group dev pytest -q

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