Add MCP server endpoint for external AI assistants (#1051)

* Add MCP server endpoint for external AI assistants

Expose Sure's Assistant::Function tools via JSON-RPC 2.0 at POST /mcp,
enabling external AI clients (Claude, GPT, etc.) to query financial data
through the Model Context Protocol.

- Bearer token auth via MCP_API_TOKEN / MCP_USER_EMAIL env vars
- JSON-RPC 2.0 with proper id threading, notification handling (204)
- Transient session (sessions.build) to prevent impersonation leaks
- Centralize function_classes in Assistant module
- Docker Compose example with Pipelock forward proxy
- 18 integration tests with scoped env (ClimateControl)

* Update compose for full Pipelock MCP reverse proxy integration

Use Pipelock's --mcp-listen/--mcp-upstream flags (PR #127) to run
bidirectional MCP scanning in the same container as the forward proxy.
External AI clients connect to port 8889, Pipelock scans requests
(DLP, injection, tool policy) and responses (injection, tool poisoning)
before forwarding to Sure's /mcp endpoint.

This supersedes the standalone compose in PR #1050.

* Fix compose --preset→--mode, add port 3000 trust comment, notification test

Review fixes:
- pipelock run uses --mode not --preset (would prevent stack startup)
- Document port 3000 exposes /mcp directly (auth still required)
- Add version requirement note for Pipelock MCP listener support
- Add test: tools/call sent as notification does not execute

app/models/assistant.rb

@@ -20,6 +20,18 @@ module Assistant
       REGISTRY.keys
     end
 
+    def function_classes
+      [
+        Function::GetTransactions,
+        Function::GetAccounts,
+        Function::GetHoldings,
+        Function::GetBalanceSheet,
+        Function::GetIncomeStatement,
+        Function::ImportBankStatement,
+        Function::SearchFamilyFiles
+      ]
+    end
+
     private
 
       def implementation_for(chat)