Add resources/subscribe and notifications/resources/updated passthrough

[bill-doccla]

Summary

mcpmu does not support resources/subscribe or notifications/resources/updated from the MCP spec. When an upstream server declares subscribe: true in its resources capability, mcpmu cannot forward subscription requests or relay update notifications to the client.

What's missing

Method	MCP Spec	mcpmu
resources/subscribe	Optional (when server declares subscribe: true)	Not implemented
resources/unsubscribe	Goes with subscribe	Not implemented
notifications/resources/updated	Server→client notification when subscribed resource content changes	Not forwarded

What this means

• If an upstream server's resource content changes mid-session (e.g. a file on disk is edited, a database view updates), the client won't be notified
• The client would need to manually re-read the resource to get fresh content
• Static resources (like oth://api-guide) are unaffected

Implementation notes

• mcpmu should check upstream server capabilities for resources.subscribe: true during initialization
• On resources/subscribe from client: forward to the correct upstream server (using the resource map from resources/list)
• On notifications/resources/updated from upstream: relay to the client with the original URI
• On resources/unsubscribe: forward to upstream and clean up tracking
• mcpmu should only advertise subscribe: true in its own capabilities if at least one active upstream server supports it

Priority

Low — no current upstream servers in the default config use resource subscriptions. Becomes relevant when proxying servers with dynamic resources (file watchers, live data feeds, etc).