Marketplace install fails on non-ephemeral self-hosted runners: 'already installed'

## Problem

When using `claude-code-action` on **non-ephemeral self-hosted runners**, the marketplace install step fails on the second run because the state from the previous run persists in `~/.claude/`:

```
✘ Failed to add marketplace: Marketplace 'claude-code-plugins' is already installed.
Please remove it first using '/plugin marketplace remove claude-code-plugins' if you want to re-install it.
```

## Expected behavior

The action should handle "already installed" gracefully — either skip the install if the marketplace is already present, or remove and re-install it.

## Reproduction

1. Use a non-ephemeral self-hosted runner
2. Configure the action with `plugin_marketplaces` and `plugins`
3. First run succeeds
4. Second run on the same runner fails

## Workaround

None that's safe for concurrent jobs. Deleting `~/.claude` between runs risks breaking concurrent jobs on the same runner.

## Environment

- Runner: self-hosted, non-ephemeral (GCP MIG)
- Action version: `b113f49a56229d8276e2bf05743ad6900121239c`
- Config: `plugin_marketplaces: 'https://github.com/anthropics/claude-code.git'`