Secure extension install

When publishing extension/server install options (especially deep-links or Install buttons), default to a secure-by-default path: require explicit/manual installation for binary-first or otherwise security-sensitive extensions. Only enable an automated install/deeplink if the target is explicitly allowlisted.

Apply it like this:

• Disable the install button/deeplink for these entries (e.g., set show_install_link to false).
• Replace it with clear manual installation instructions.
• If an Install button is required, add/extend the deeplink allowlist in deeplink.ts with a narrow entry for the exact server/extension (no broad or dynamic matching).

Example (pattern):

{
  "id": "nika",
  "name": "Nika",
  "description": "...",
  "command": "nika mcp",
  "show_install_link": false
}

Then document manual install, e.g.:

• goose session --with-extension "nika mcp"
• Settings → Extensions