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();