Migrate MCP to 2.0 (no `initialize`, sessions via static handles)

[cstns]

We should move our MCP over to 2.0. The main difference is there's no initialize step any more, and sessions aren't negotiated at connect time, they come from static handles instead.

Places this touches:

• the MCP tool gateway
• forge, probably, since that's where the handles would come from

Open questions before anyone starts:

• do we keep 1.x working for a while, or just cut over?
• the browser-session flow assumes a live session today, does that still hold with static handles, or does it need rethinking?