feat: display detailed error messages when prepare step fails (#82)

* feat: display detailed error messages when prepare step fails

- Capture prepare step errors in action.yml (up to 2000 chars)
- Add error details to comment update with collapsible section
- Handle both prepare and Claude execution failures separately
- Add test coverage for error detail display

This helps users debug issues like git errors, permission problems,
and branch creation failures more easily.

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

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

* refactor: simplify error capture to show clean error messages only

- Remove complex shell script that captured full output logs
- Use core.setOutput in prepare.ts to pass clean error message directly
- Avoid exposing potentially sensitive information from logs
- Show only the actual error message (e.g. 'Failed to fetch issue data')

This provides cleaner, more readable error messages without the risk
of exposing sensitive information from debug logs.

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

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

* refactor: simplify error display to show clean error messages only

- Remove collapsible <details> section for error messages
- Display errors in simple code blocks since messages are now clean and short
- Makes error messages more direct and readable

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

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

---------

Co-authored-by: Claude <noreply@anthropic.com>

src/entrypoints/prepare.ts

@@ -92,7 +92,10 @@ async function run() {
     );
     core.setOutput("mcp_config", mcpConfig);
   } catch (error) {
-    core.setFailed(`Prepare step failed with error: ${error}`);
+    const errorMessage = error instanceof Error ? error.message : String(error);
+    core.setFailed(`Prepare step failed with error: ${errorMessage}`);
+    // Also output the clean error message for the action to capture
+    core.setOutput("prepare_error", errorMessage);
     process.exit(1);
   }
 }