Feature request: pluginDefaultScope setting to control default install scope

Summary

Add a pluginDefaultScope setting to settings.json that controls the default scope when installing plugins via /plugin.

Motivation

Currently, plugin install scope (user vs project) is inferred from context — user when outside a project root, project when inside one. This works most of the time, but there is no way to explicitly enforce project-scoped installs as a default across all contexts.

A pluginDefaultScope setting would let users (and admins via managed settings) enforce a consistent default without relying on directory context.

Proposed Schema Addition

"pluginDefaultScope": {
  "description": "Default scope for plugin installation: 'user' installs globally, 'project' scopes to the current working directory (default: inferred from context)",
  "type": "string",
  "enum": ["user", "project"]
}

Example Usage

// ~/.claude/settings.json
{
  "pluginDefaultScope": "project"
}

With this set, /plugin install <name> would default to project scope even when the project root detection is ambiguous.

Workaround

Currently none — the field is rejected by schema validation with Unrecognized field: pluginDefaultScope.

Environment

• Claude Code version: latest (claude-sonnet-4-6)
• Platform: macOS Darwin 25.4.0