Add plugins input to GitHub Action

This commit adds support for installing Claude Code plugins via a new `plugins` input parameter.

Changes:
- Added `plugins` input to action.yml (comma-separated list)
- Created `install-plugins.ts` with plugin installation logic
- Added comprehensive tests in `install-plugins.test.ts`
- Updated base-action index.ts to call plugin installation
- Plugins are installed after settings setup but before Claude execution

Usage example:
```yaml
- uses: anthropic-ai/claude-code-action@main
  with:
    plugins: "feature-dev,test-coverage-reviewer"
```

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

base-action/src/index.ts

@@ -5,6 +5,7 @@ import { preparePrompt } from "./prepare-prompt";
 import { runClaude } from "./run-claude";
 import { setupClaudeCodeSettings } from "./setup-claude-code-settings";
 import { validateEnvironmentVariables } from "./validate-env";
+import { installPlugins } from "./install-plugins";
 
 async function run() {
   try {
@@ -15,6 +16,12 @@ async function run() {
       undefined, // homeDir
     );
 
+    // Install plugins if specified
+    await installPlugins(
+      process.env.INPUT_PLUGINS,
+      process.env.INPUT_PATH_TO_CLAUDE_CODE_EXECUTABLE || "claude",
+    );
+
     const promptConfig = await preparePrompt({
       prompt: process.env.INPUT_PROMPT || "",
       promptFile: process.env.INPUT_PROMPT_FILE || "",