chore: adds issue creator agent

[GenDelta]

This pull request introduces a new agent specification for the GitHub Copilot system. The agent, named "Issue Creator Agent," is designed to process user prompts and generate structured GitHub issues for the codexsit/codexsit.github.io repository. The main focus is on ensuring issues are actionable, well-structured, and consistently formatted.

Agent definition and workflow:

• Added .github/agents/issue_creator.agent.md to define the "Issue Creator Agent," which is responsible for analyzing user requests and creating structured GitHub issues in the codexsit/codexsit.github.io repository.

Instructions and constraints:

• Outlined a strict workflow for the agent, including analyzing the request, structuring issue content with "Requirements" and "Acceptance Criteria" sections, drafting a concise title, and creating the issue using GitHub tools.
• Specified an output format for drafting issues, enforcing the inclusion of both "Requirements" (bulleted list) and "Acceptance Criteria" (checklist) sections.
• Added constraints to ensure the agent never creates issues without the required sections, always targets the correct repository, and does not solve issues directly with code unless explicitly asked.