Migrate MCP server to the official rmcp SDK

[getappz]

Replaces the hand-rolled JSON-RPC MCP stdio server with the official rmcp 0.16 crate (modelcontextprotocol/rust-sdk). Feature set is unchanged: 2 tools (get_routing_suggestion, check_session_health) and 2 resources (agentflare://sessions, agentflare://nudges), with all business logic reused verbatim from crate::optimize.

Implementation notes:

• rmcp features [server, transport-io]; tokio kept minimal ([rt, macros, io-util, io-std]) with an isolated current-thread runtime scoped to the mcp subcommand only — every other subcommand stays fully synchronous.
• Tools via #[tool_router]/#[tool] macros; resources via a #[tool_handler] impl ServerHandler with sync helpers (list_resources_sync/read_resource_sync) extracted for unit-testability.
• serverInfo explicitly reports agentflare's own name/version (rmcp's Default would report rmcp/0.16.0).
• Cargo.toml description updated: the crate is no longer zero-dependency.

Two intentional client-observable behavior changes:

1. Unknown resource URIs now return a proper JSON-RPC resource_not_found error (old server returned a fake-successful text/plain placeholder).
2. Invalid tool arguments (e.g. empty session_id, missing prompt) now surface as protocol-level invalid_params (-32602) errors instead of tool-result-level isError payloads; rmcp also schema-validates arguments.

Verification: 94 tests passing (cargo test -- --test-threads=1); manual end-to-end stdio session exercising initialize + handshake, both resources, a bogus URI, tools/list, and both tools (7/7 expected responses, confirmed serverInfo identity).

[github-actions]

All contributors have signed the CLA. ✅
_{Posted by the CLA Assistant Lite bot.}