feat: implement Claude Code GitHub Action v1.0 with auto-detection and slash commands

Major features:
- Mode auto-detection based on GitHub event type
- Unified prompt field replacing override_prompt and direct_prompt
- Slash command system with pre-built commands
- Full backward compatibility with v0.x

Key changes:
- Add mode detector for automatic mode selection
- Implement slash command loader with YAML frontmatter support
- Update action.yml with new prompt input
- Create pre-built slash commands for common tasks
- Update all tests for v1.0 compatibility

Breaking changes (with compatibility):
- Mode input now optional (auto-detected)
- override_prompt deprecated (use prompt)
- direct_prompt deprecated (use prompt)

test/install-mcp-server.test.ts

@@ -25,6 +25,7 @@ describe("prepareMcpConfig", () => {
     isPR: false,
     inputs: {
       mode: "tag",
+      prompt: "",
       triggerPhrase: "@claude",
       assigneeTrigger: "",
       labelTrigger: "",