unified-mcp-server: add a streamable-HTTP transport behind MCP_HTTP_PORT #410

Part of #408. Depends on the response-sink refactor.

Why

MCP stdio spawns one child per client by design, so the best stdio can do is one process per session. A machine-wide singleton needs a transport many clients can share.

What

Add an HTTP listener behind an env flag (MCP_HTTP_PORT), keeping stdio as the default so existing installs are untouched.

Must match the streamable-HTTP MCP profile Claude Code already speaks — it connects to HTTP MCP servers with {"type":"http","url":...}, SSE-framed replies and an mcp-session-id header. Evidence it works: the daintree entry in a real ~/.claude.json is exactly this shape.

Note http/https are already imported in the server, but only for outbound provider health checks (transport.request at :498 and :579) — there is no listener today.

Acceptance

• MCP_HTTP_PORT=N starts a listener; unset keeps pure stdio
• initialize / tools/list / tools/call all answer over HTTP
• replies are SSE-framed and mcp-session-id is honoured
• a real Claude Code session can connect with {"type":"http","url":...} and call a provider