MCP parity for Moments: `update_moment` + `destroy_moment` tools

update? and destroy? added to MomentPolicy (DM-only). UpdateMomentTool accepts id + optional content/kind; runs WikiLinkResolver on content. DestroyMomentTool accepts id. No re-broadcast on edit (moments are live-session artifacts). 4 new policy tests; full suite 495 green. 132 tools total.