Bug: Marketplace cloning fails with 'repository not found' for private/internal repos - git authentication not configured

## Description

When using `plugin_marketplaces` with a private or internal GitHub repository, marketplace cloning fails with:

```
✘ Failed to add marketplace: Failed to clone marketplace repository: HTTPS authentication failed. Ensure your token has access to this repository.
Original error: Cloning into '/home/runner/.claude/plugins/marketplaces/temp_1769074856991'...
remote: Repository not found.
fatal: repository 'https://github.com/org/private-marketplace.git/' not found
```

**This occurs even when:**
- `GITHUB_TOKEN` is properly set in the environment
- The GitHub App is installed on the marketplace repository
- The token has verified access to the repository

## Steps to Reproduce

1. Create a private/internal marketplace repository (e.g., `org/private-marketplace`)
2. Install the Claude GitHub App on both repositories (main repo + marketplace)
3. Configure workflow with:
```yaml
- uses: anthropics/claude-code-action@v1
with:
anthropic_api_key: ${{ secrets.ANTHROPIC_API_KEY }}
plugin_marketplaces: "https://github.com/org/private-marketplace.git"
```
4. Trigger the workflow via `@claude` comment
5. Observe marketplace clone failure

## Expected Behavior

The action should successfully clone private marketplace repositories using the `GITHUB_TOKEN` obtained via OIDC or provided via `github_token` input.

## Actual Behavior

Git fails to authenticate during `git clone`, reporting "repository not found" (GitHub's standard error for permission denied).

## Root Cause

Investigation of the codebase reveals:

1. **Git authentication IS configured** in `src/github/operations/git-config.ts` (`configureGitAuth()`)
2. **BUT** this function is only called in agent/tag modes
3. **Marketplace installation** happens in `base-action/src/index.ts` → `installPlugins()` → spawns `claude plugin marketplace add `
4. **Git has no credential helper** configured at this point → authentication fails

**Relevant code locations:**
- `base-action/src/install-plugins.ts:191-201` - marketplace cloning
- `src/github/operations/git-config.ts:22-61` - git auth function (unused in base-action)
- `base-action/src/index.ts:10-24` - entry point (missing git config call)

## Proposed Solution

Call `configureGitAuth()` (or equivalent) in `base-action/src/index.ts` **before** `installPlugins()`:

```typescript
import { configureGitAuth } from '../src/github/operations/git-config';

// In base-action/src/index.ts
const token = process.env.GITHUB_TOKEN || process.env.GH_TOKEN;
if (token) {
await configureGitAuth(token); // Configure git before installing plugins
}

await installPlugins(plugins, marketplaces);
```

Or configure git credentials directly:
```typescript
execSync(`git config --global credential.helper '!f() { echo "username=x-access-token"; echo "password=${token}"; }; f'`);
```

## Environment

- Action version: `a017b830c03e23789b11fb69ed571ea61c12e45c` (2026-01-16)
- Marketplace visibility: `INTERNAL` (GitHub Enterprise)
- Runner: `ubuntu-latest`

## Impact

This prevents organizations from using private plugin marketplaces without creating additional PATs, reducing security (more tokens to manage) and convenience.