feat: ship slash commands with GitHub Action (#381)

* feat: add slash command shipping infrastructure

- Created /slash-commands/ directory to store bundled slash commands
- Added code-review.md slash command for automated PR reviews
- Modified setup-claude-code-settings.ts to copy slash commands to ~/.claude/
- Added test coverage for slash command installation
- Commands are automatically installed when the GitHub Action runs

* fix: simplify slash command implementation to match codebase patterns

- Reverted to using Bun's $ shell syntax consistently with the rest of the codebase
- Simplified slash command copying to basic shell commands
- Removed unnecessary fs/promises complexity
- Maintained all functionality and test coverage
- More appropriate for GitHub Action context where inputs are trusted

* remove test slash command

* fix: rename slash_commands_dir to experimental_slash_commands_dir

- Added 'experimental' prefix as suggested by Ashwin
- Updated all references in action.yml and base-action
- Restored accidentally removed code-review.md file

---------

Co-authored-by: km-anthropic <km-anthropic@users.noreply.github.com>

base-action/src/index.ts

@@ -10,7 +10,11 @@ async function run() {
   try {
     validateEnvironmentVariables();
 
-    await setupClaudeCodeSettings(process.env.INPUT_SETTINGS);
+    await setupClaudeCodeSettings(
+      process.env.INPUT_SETTINGS,
+      undefined, // homeDir
+      process.env.INPUT_EXPERIMENTAL_SLASH_COMMANDS_DIR,
+    );
 
     const promptConfig = await preparePrompt({
       prompt: process.env.INPUT_PROMPT || "",