Describe the bug/feature
The plugin_marketplaces is currently very strict and requires the marketplace and plugin to be in separate git repos, through this REGEX check.

This means if you have Claude plugins and marketplaces within the same directory, it won't be able to made available in Claude Code Github Action.

To Reproduce
Suppose you have this plugin marketplace

my-first-plugin/
├── .claude-plugin/
│   └── plugin.json          # Plugin metadata
├── commands/                 # Custom slash commands (optional)
    └── hello.md

Which you want Claude Code Github Action to be have access to, for example separate commands and such.

plugin_marketplaces: "./my-first-marketplace"
plugins: "my-plugin@my-first-marketplace"

Expected behavior
I would expect Claude Code to be able to install this local plugin, instead of erroring out and complaining that the URL is unsupported.

Action failed with error: Error: Invalid marketplace URL format: ./my-first-marketplace

Workflow yml file
Any workflow file that links to a local marketplace

plugin_marketplaces: "./my-first-marketplace"
plugins: "my-plugin@my-first-marketplace"