feat(blender): agent control via mcp-for-blender (in-container, ssh stdio)

The MCP server (mcp-for-blender 2.1.1, frozen requirements) runs inside the Blender
container. Its add-on is vendored at upstream 41a18432 (MIT) and started by a
startup hook. scripts/blender-mcp carries the stdio over ssh + docker exec, so the
add-on socket, which runs arbitrary Python with no auth, stays on the container's
localhost with no published port. It also runs there because viewport screenshots
need a filesystem shared by server and Blender. Telemetry is off and safe mode is
on. The hook also defaults Cycles to OptiX on GPU 3, because safe mode forbids
agents from touching preferences.

Verified end to end from nh3-dev: 36 tools; a GPU render of an agent-built scene;
a viewport screenshot; and safe mode refusing 'import os'. Blender left down
(on demand).

stacks/blender/conf/scripts/addons/LICENSE-mcp-for-blender

@@ -0,0 +1,21 @@
+MIT License
+
+Copyright (c) 2025 Siddharth Ahuja
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
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+in the Software without restriction, including without limitation the rights
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+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions:
+
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