Remove unsafe MCP skill install tool

[elimaximus1]

Removes the write-capable install_skill MCP tool, which accepted client-supplied skill and file paths and could write outside the intended skills directory.

Validation:

• python -m py_compile openbb_platform/extensions/mcp_server/openbb_mcp_server/app/app.py
• python -m pytest tests
• python -m ruff check openbb_mcp_server/app/app.py

[CLAassistant]

All committers have signed the CLA.

[deeleeramone]

The user is the server owner, who also is running it over localhost while controlling the client. You're really only a danger to yourself.