docs: describe the MCP server and panel in the security guide

SECURITY.md gains a section on what /mcp accepts, the panel's request key,
the browser keys the panel build carries and embed-mode framing. TOOLS.md
now describes what panel_request may request, and the changelog records
the recent MCP and embed changes.

Co-Authored-By: Claude Opus 5.5 (1M context) <[email protected]>

CHANGELOG.md

@@ -2,6 +2,18 @@
 
 ## [Unreleased]
 
+- Tighten the local MCP server and embed mode. The panel's `panel_request`
+  needs the key in its MCP server's panel page, and refuses Provider
+  Settings, credential and model endpoints, `/mcp` and development server
+  routes in any letter case or encoding; panels on one server share six
+  requests in flight, and further requests wait their turn. `/mcp` applies Provider Settings' proxy and sharing checks,
+  requires a Host on the port it reached and an Origin from that host, and
+  times out request bodies after 30 seconds; stdio honors
+  `notifications/cancelled`. Embed mode is no longer framable unless
+  `GEV_EMBED_FRAME_ANCESTORS` allows the framing page, and view answers go
+  only to the origin that sent the view. SECURITY.md describes the MCP surface
+  and the panel's browser keys.
+
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