feat: send user request as separate content block for slash command support (#785)

* feat: send user request as separate content block for slash command support

When in tag mode with the SDK path, extracts the user's request from the
trigger comment (text after @claude) and sends it as a separate content
block. This enables the CLI to process slash commands like "/review-pr".

- Add extract-user-request utility to parse trigger comments
- Write user request to separate file during prompt generation
- Send multi-block SDKUserMessage when user request file exists
- Add tests for the extraction utility

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

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

* fix: address PR feedback

- Fix potential ReDoS vulnerability by using string operations instead of regex
- Remove unused extractUserRequestFromEvent function and tests
- Extract USER_REQUEST_FILENAME to shared constants
- Conditionally log user request based on showFullOutput setting
- Add JSDoc documentation to extractUserRequestFromContext

---------

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

src/utils/extract-user-request.ts

@@ -0,0 +1,32 @@
+/**
+ * Extracts the user's request from a trigger comment.
+ *
+ * Given a comment like "@claude /review-pr please check the auth module",
+ * this extracts "/review-pr please check the auth module".
+ *
+ * @param commentBody - The full comment body containing the trigger phrase
+ * @param triggerPhrase - The trigger phrase (e.g., "@claude")
+ * @returns The user's request (text after the trigger phrase), or null if not found
+ */
+export function extractUserRequest(
+  commentBody: string | undefined,
+  triggerPhrase: string,
+): string | null {
+  if (!commentBody) {
+    return null;
+  }
+
+  // Use string operations instead of regex for better performance and security
+  // (avoids potential ReDoS with large comment bodies)
+  const triggerIndex = commentBody
+    .toLowerCase()
+    .indexOf(triggerPhrase.toLowerCase());
+  if (triggerIndex === -1) {
+    return null;
+  }
+
+  const afterTrigger = commentBody
+    .substring(triggerIndex + triggerPhrase.length)
+    .trim();
+  return afterTrigger || null;
+}