feat: merge prepare and run steps into unified entry point

Consolidates the two-step architecture (prepare → run) into a single
step to eliminate file-based and output-based communication.

Changes:
- Add base-action/src/lib.ts with exports for main action import
- Modify run-claude-sdk.ts to accept prompt string directly and return result
- Add generatePromptContent() that returns prompt without file I/O
- Update mode prepare() to return promptContent in result
- Create src/entrypoints/run.ts as unified entry point
- Update action.yml to use single Run Claude Code step
- Update output references from steps.prepare to steps.claude-code

Benefits:
- No file I/O for prompt - data stays in memory
- No step output parsing - direct function returns
- Simpler debugging - single entry point
- Faster execution - no subprocess overhead
- Type safety - TypeScript across the boundary

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

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

src/modes/types.ts

@@ -97,4 +97,10 @@ export type ModeResult = {
     currentBranch: string;
   };
   mcpConfig: string;
+  /** Generated prompt content for Claude */
+  promptContent?: string;
+  /** Comma-separated list of allowed tools */
+  allowedTools?: string;
+  /** Comma-separated list of disallowed tools */
+  disallowedTools?: string;
 };