docs: link ICP MCP from the service discoverability guide

[michaelmorandi]

Summary

• Adds a short "Connecting an agent" section to the service discoverability guide (placed after the acceptance tests) noting that an MCP-capable assistant can consume the guide's five layers through the DFINITY-hosted ICP MCP connector, linking https://internetcomputer.org/icp-mcp/#enable.

Observed spec-vs-implementation divergence (informational)

While testing the hosted MCP (mcp.internetcomputer.org) against an app that implements this guide end to end, we observed two places where the connector diverges from the guide:

1. discover_app_canisters sources the app's composition from env.json, the JS bundle, response headers, and /.well-known/ic-app.json, rather than this guide's /.well-known/ic-architecture manifest (Layer 1).
2. resolve_app does not consult /.well-known/ii-derivation-origin (Layer 5). It assumed the visible custom domain as the derivation origin, which derives a wrong principal for apps that pin a canonical origin per Layer 5.

Everything else in the guide works end to end through the connector: candid:service, getApiDoc served verbatim, and per-user OQL reads once the correct derivation origin is supplied.

Suggestion: the MCP could adopt these two well-known files as authoritative sources so the guide and its flagship consumer agree. Happy to share the test details if useful.

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