[Feature] agents: MCP server — expose actor tools over MCP (SDK-free)

[pathosDev]

Part of the agents epic #421.

Rationale

The inverse direction of the MCP client: expose selected tools — or whole agents — of an actor system as an MCP server, so external agent hosts can call into it. Same in-house JSON-RPC layer as the client issue; no SDK requirement.

Scope

• mcpRoutes(registry, { expose, auth }) plugging into the HTTP DSL (streamable HTTP transport), reusing BearerTokenAuth; plus a stdio runner for CLI embedding.
• Incoming tools/call → ToolRegistry.invoke (schema-validated, traced, supervised — same path as internal calls).
• askAgentTool(agentRef, definition): expose a whole agent as a single MCP tool.

const routes = concat(
  mcpRoutes(agents.tools, { expose: ['get_weather'], auth: BearerTokenAuth(config) }),
  apiRoutes,
);

Documentation

"MCP" docs page (server half, EN + DE) incl. security guidance (auth required by default outside dev); JSDoc; CHANGELOG.

Acceptance

• End-to-end test: an MCP client fixture lists and calls an exposed tool over HTTP with auth; unauthorized calls rejected; stdio runner smoke-tested.

Non-goals

MCP resources/prompts; multi-tenant authorization schemes beyond the existing middleware.

Relates

Epic #421; depends on #423 (ToolRegistry) and shares the JSON-RPC layer with the MCP client issue. Dependency policy #419.