feat: Add plugin_dir parameter support for Claude Code workers

[mzdbxqh]

Summary

Add plugin_dir parameter support to allow specifying custom plugin directories when creating Claude Code workers.

Changes

• Add plugin_dir field to WorkerConfig TypedDict
• Update AgentCLI interface to accept plugin_dir parameter
• Implement --plugin-dir flag support in claude.py
• Add plugin_dir parameter to codex.py (ignored, as Codex doesn't support it)
• Update start_agent_in_session in all backends to pass plugin_dir
• Update spawn_workers to read and pass plugin_dir from worker config

Testing

• ✅ Command generation test passed
• ✅ Worker creation test passed
• ✅ Plugin loading verified (68 skills loaded with glaf4: namespace)
• ✅ Manual testing with Claude Code confirmed plugin_dir works correctly

Use Case

Enables testing Claude Code plugins (like GLAF4) by specifying custom plugin directories when spawning workers.

Example

mcporter call maniple.spawn_workers \
  workers='[{
    "project_path": "/path/to/project",
    "plugin_dir": "/path/to/plugin",
    "skip_permissions": true
  }]'