feat: support local plugin marketplace paths

[gor-st]

Summary

• Adds support for local filesystem paths in the plugin_marketplaces input
• Allows users to use ./my-marketplace, ../shared-plugins, /absolute/path, and Windows paths like C:\plugins
• Local paths are passed directly to Claude Code which handles them appropriately

Problem

Since claude-code-action does not allow multi-repo access, it's impossible to add your team's plugin marketplaces from other repositories directly via HTTPS URLs. Even if you checkout the marketplace repository yourself, the action only accepts HTTPS git URLs.

Solution

This PR enables users to:

1. Checkout their company's marketplace repository in a prior workflow step
2. Add it as a local directory path (e.g., ./my-marketplace)

Claude Code natively supports local paths for plugin marketplaces, so the action should support them as well.

Example Usage

- name: Checkout marketplace
  uses: actions/checkout@v4
  with:
    repository: my-org/claude-plugins-marketplace
    path: ./my-marketplace

- name: Run Claude
  uses: anthropics/claude-code-action@v1
  with:
    plugin_marketplaces: |
      ./my-marketplace
    plugins: |
      my-org/custom-plugin

Changes

• Added isLocalPath() function to detect local path formats
• Modified validateMarketplaceInput() to accept local paths without URL validation
• Added 8 new tests covering various local path scenarios

Test plan

• All existing tests pass (45 tests)
• New tests for local path scenarios added and passing
• Tests cover ./, ../, /, Windows paths, and mixed local/remote scenarios

Fixes #664

🤖 Generated with Claude Code

[gor-st]

@ashwin-ant could you review PR #761 when you have a moment?

This is a missing piece for enterprise integrations - private organizations typically maintain their own plugin marketplaces in internal repositories, and without local path support, there's no way to use them with the action.

Here is the original github issue: #664

[gor-st]

I tested this, and its working

[gor-st]

@ashwin-ant would you mind throwing a look, please?

[ashwin-ant]

Thanks!