feat: SSE MCP transport — shared state with proxy on :9090

[alderpath]

Problem

The MCP server was stdio-only — a separate subprocess with its own memory. No shared state with the proxy (anti-decision, session hashes, response cache). Each agent needed its own subprocess.

Solution

SSE MCP runs on the same axum server as the proxy (port :9090), sharing the same process memory.

Architecture

Agent → GET /mcp/sse → SSE connection established
Agent ← event: endpoint /mcp/messages?sessionId=xxx
Agent → POST /mcp/messages?sessionId=xxx (JSON-RPC)
Agent ← event: message (JSON-RPC response via SSE)

Shared State

SSE MCP tools and proxy share the same memory space:

• Anti-decision DB — heal tool knows past failure patterns
• Session hashes — search ranked by session co-occurrence
• Response cache — MCP tool results cached alongside proxy responses
• Single port (:9090) for everything

Transport Options

| Transport | Config | When |
|---|---|---|
| SSE | url: http://127.0.0.1:9090/mcp/sse | init default (shared state) |
| Stdio | command: reliary-agent mcp | Always-available fallback |

Safety

• Sessions auto-expire after 5 min idle
• SSE disconnect cleans up immediately
• tokio mpsc channels: no blocking, no deadlocks
• Stdio stays forever as fallback

Code Changes

• mcp.rs: refactored — extracted pure dispatch_tool_call() + tool_definitions() for reuse
• mcp_sse.rs: new (~170 lines) — SSE handler + messages handler
• proxy.rs: 2 new routes wired (/mcp/sse, /mcp/messages)
• init.rs: inject_sse_mcp_server() — URL-based MCP config

Tests

86 tests pass (0 regressions). Zero build warnings.