mcp: live-refresh global connector access without restart; add MCP list to kid agent

A user's session sees global MCP connectors through UserMcpView, filtered by
accessible_global — a snapshot of mcp_global_access taken when the user's
UserContext is built at login. That context is cached until restart, so an admin
enabling/deleting a global connector or changing its access set was invisible in
MCP_LIST (and in the tool surface) until the whole process restarted.

Make accessible_global a swappable cell (SharedGlobalAccess, the MCP twin of
SharedFs for §6 fs remount): UserContext::refresh_global_access re-reads the
registry and stores it in place, and Skald::refresh_global_mcp_access broadcasts
that to every live context. Wire it into global_enable, global_delete,
global_set_access and user_connectors_set so a grant/enable is reflected in
running sessions immediately.

Also add the shared common/mcp.md include (the <!-- MCP_LIST --> sentinel) to the
kid agent, aligning it with the other agents.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

agents/kid/AGENT.md

@@ -73,6 +73,10 @@ There may be other helpers in the household's team — each good at different th
 
 ---
 
+<!-- INCLUDE: common/mcp.md -->
+
+---
+
 ## Shared folders
 
 Shared folders are special places where some members of the household can read and write the same files together — photo albums, a family story, a playlist. You reach them at `shared/{name}/…`. Your folders, who else can see each one, and what each is for: