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/update-comment-link.ts

@@ -145,38 +145,48 @@ async function run() {
       duration_api_ms?: number;
     } | null = null;
     let actionFailed = false;
+    let errorDetails: string | undefined;
 
-    // Check for existence of output file and parse it if available
-    try {
-      const outputFile = process.env.OUTPUT_FILE;
-      if (outputFile) {
-        const fileContent = await fs.readFile(outputFile, "utf8");
-        const outputData = JSON.parse(fileContent);
+    // First check if prepare step failed
+    const prepareSuccess = process.env.PREPARE_SUCCESS !== "false";
+    const prepareError = process.env.PREPARE_ERROR;
 
-        // Output file is an array, get the last element which contains execution details
-        if (Array.isArray(outputData) && outputData.length > 0) {
-          const lastElement = outputData[outputData.length - 1];
-          if (
-            lastElement.role === "system" &&
-            "cost_usd" in lastElement &&
-            "duration_ms" in lastElement
-          ) {
-            executionDetails = {
-              cost_usd: lastElement.cost_usd,
-              duration_ms: lastElement.duration_ms,
-              duration_api_ms: lastElement.duration_api_ms,
-            };
+    if (!prepareSuccess && prepareError) {
+      actionFailed = true;
+      errorDetails = prepareError;
+    } else {
+      // Check for existence of output file and parse it if available
+      try {
+        const outputFile = process.env.OUTPUT_FILE;
+        if (outputFile) {
+          const fileContent = await fs.readFile(outputFile, "utf8");
+          const outputData = JSON.parse(fileContent);
+
+          // Output file is an array, get the last element which contains execution details
+          if (Array.isArray(outputData) && outputData.length > 0) {
+            const lastElement = outputData[outputData.length - 1];
+            if (
+              lastElement.role === "system" &&
+              "cost_usd" in lastElement &&
+              "duration_ms" in lastElement
+            ) {
+              executionDetails = {
+                cost_usd: lastElement.cost_usd,
+                duration_ms: lastElement.duration_ms,
+                duration_api_ms: lastElement.duration_api_ms,
+              };
+            }
           }
         }
-      }
 
-      // Check if the action failed by looking at the exit code or error marker
-      const claudeSuccess = process.env.CLAUDE_SUCCESS !== "false";
-      actionFailed = !claudeSuccess;
-    } catch (error) {
-      console.error("Error reading output file:", error);
-      // If we can't read the file, check for any failure markers
-      actionFailed = process.env.CLAUDE_SUCCESS === "false";
+        // Check if the Claude action failed
+        const claudeSuccess = process.env.CLAUDE_SUCCESS !== "false";
+        actionFailed = !claudeSuccess;
+      } catch (error) {
+        console.error("Error reading output file:", error);
+        // If we can't read the file, check for any failure markers
+        actionFailed = process.env.CLAUDE_SUCCESS === "false";
+      }
     }
 
     // Prepare input for updateCommentBody function
@@ -189,6 +199,7 @@ async function run() {
       prLink,
       branchName: shouldDeleteBranch ? undefined : claudeBranch,
       triggerUsername,
+      errorDetails,
     };
 
     const updatedBody = updateCommentBody(commentInput);