Feature request: support local plugin path for in-development skills

Problem

When developing a new plugin skill (a .md command file), there is no way to point Claude Code at the local working directory for testing. The plugin system reads exclusively from the marketplace cache at ~/.claude/plugins/marketplaces/.

This means the only way to test a skill under development is to manually copy the file to the cache directory after every change:

cp plugins/my-toolkit/commands/my-skill.md \
  ~/.claude/plugins/marketplaces/my-marketplace/plugins/my-toolkit/commands/my-skill.md

Running /reload-plugins refreshes from the cache — it does not pull from the source repo.

Proposed solution

Support a local override path in plugin configuration (e.g. in .claude/settings.json or the plugin manifest) that points Claude Code at the working directory instead of the cache for a given plugin. Something like:

{
  "plugins": {
    "my-toolkit": {
      "source": "local",
      "path": "./plugins/my-toolkit"
    }
  }
}

Or a CLI flag: claude --plugin-dev-path ./plugins/my-toolkit

Why it matters

Without this, the development loop for skills is:

1. Edit file in repo
2. Manually copy to cache
3. Restart Claude Code session
4. Test

This makes iterating on skills during PR review cumbersome and error-prone (easy to test a stale cached version without realizing it).