fix: error when skill/exclude filters are used with native install mode

[christso]

Summary

When a plugin uses install: native, allagents delegates installation to the client's native plugin system (e.g., claude plugin install). Native plugin systems install plugins as whole units with no per-artifact filtering — you get all skills, hooks, commands, agents, and MCP servers, or none.

However, allagents currently allows users to configure skills and exclude filters on plugins that use native install mode. These filters silently do nothing, which is misleading.

plugins:
  - source: superpowers@claude-plugins-official
    install: native
    skills:
      - test-driven-development  # ← has no effect in native mode
    exclude:
      - "hooks/**"               # ← has no effect in native mode

Expected behavior

TUI (interactive mode): Grey out / hide the skill selection UI when native install mode is selected. Users shouldn't be offered choices that have no effect.

Headless mode: Error with a message like: "skill/exclude filters are not supported for native plugins. Change to install: file to use filters."

Context

Validated by testing claude plugin disable/enable — Claude Code's native plugin system only supports whole-plugin enable/disable via settings.json (enabledPlugins map). There is no per-skill, per-hook, per-command, or per-agent toggle.

Similarly, npx plugins (open-plugin format) only supports whole-plugin add/remove with no artifact-level control.