fix: address PR #683 review feedback

Critical fixes:
- Remove duplicate core.setFailed() call in parseAndSetStructuredOutputs
  (fixes double error reporting issue)
- Extract JSON schema handling to shared utility function
  (eliminates code duplication between agent/tag modes)

Changes:
- base-action/src/run-claude.ts: Remove redundant setFailed() before throw
- src/utils/json-schema.ts: New shared appendJsonSchemaArg() utility
- src/modes/agent/index.ts: Use shared JSON schema utility
- src/modes/tag/index.ts: Use shared JSON schema utility

All tests passing, types checked, code formatted.

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

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

src/modes/agent/index.ts

@@ -7,6 +7,7 @@ import { parseAllowedTools } from "./parse-tools";
 import { configureGitAuth } from "../../github/operations/git-config";
 import type { GitHubContext } from "../../github/context";
 import { isEntityContext } from "../../github/context";
+import { appendJsonSchemaArg } from "../../utils/json-schema";
 
 /**
  * Extract GitHub context as environment variables for agent mode
@@ -150,12 +151,7 @@ export const agentMode: Mode = {
     }
 
     // Add JSON schema if provided
-    const jsonSchemaStr = process.env.JSON_SCHEMA || "";
-    if (jsonSchemaStr) {
-      // CLI validates schema - just escape for safe shell passing
-      const escapedSchema = jsonSchemaStr.replace(/'/g, "'\\''");
-      claudeArgs += ` --json-schema '${escapedSchema}'`;
-    }
+    claudeArgs = appendJsonSchemaArg(claudeArgs);
 
     // Append user's claude_args (which may have more --mcp-config flags)
     claudeArgs = `${claudeArgs} ${userClaudeArgs}`.trim();

src/modes/tag/index.ts

@@ -15,6 +15,7 @@ import { isEntityContext } from "../../github/context";
 import type { PreparedContext } from "../../create-prompt/types";
 import type { FetchDataResult } from "../../github/data/fetcher";
 import { parseAllowedTools } from "../agent/parse-tools";
+import { appendJsonSchemaArg } from "../../utils/json-schema";
 
 /**
  * Tag mode implementation.
@@ -178,12 +179,7 @@ export const tagMode: Mode = {
     claudeArgs += ` --allowedTools "${tagModeTools.join(",")}"`;
 
     // Add JSON schema if provided
-    const jsonSchemaStr = process.env.JSON_SCHEMA || "";
-    if (jsonSchemaStr) {
-      // CLI validates schema - just escape for safe shell passing
-      const escapedSchema = jsonSchemaStr.replace(/'/g, "'\\''");
-      claudeArgs += ` --json-schema '${escapedSchema}'`;
-    }
+    claudeArgs = appendJsonSchemaArg(claudeArgs);
 
     // Append user's claude_args (which may have more --mcp-config flags)
     if (userClaudeArgs) {

src/utils/json-schema.ts

@@ -0,0 +1,17 @@
+/**
+ * Appends JSON schema CLI argument if json_schema is provided
+ * Escapes schema for safe shell passing
+ */
+export function appendJsonSchemaArg(
+  claudeArgs: string,
+  jsonSchemaStr?: string,
+): string {
+  const schema = jsonSchemaStr || process.env.JSON_SCHEMA || "";
+  if (!schema) {
+    return claudeArgs;
+  }
+
+  // CLI validates schema - just escape for safe shell passing
+  const escapedSchema = schema.replace(/'/g, "'\\''");
+  return `${claudeArgs} --json-schema '${escapedSchema}'`;
+}