Add support for creating and updating repository security advisories

Beginne damit, das vorhandene security_advisories-Toolset und seine schreibgeschützten Tools zu finden, und vergleiche dann deren Muster mit den drei verknüpften GitHub REST API-Operationen. Die Aufgabe ist erledigt, wenn das Toolset das Erstellen von Entwürfen, das Aktualisieren und Veröffentlichen von Advisories sowie das Anfordern von CVE IDs über die dokumentierten Endpunkte unterstützt.

Describe the feature or problem you'd like to solve

The current security_advisories toolset only supports reading advisories (list_repository_security_advisories, list_org_repository_security_advisories, get_global_security_advisory). There is no support for creating, updating, publishing, or requesting CVE IDs for advisories via MCP, forcing security teams to context-switch to the GitHub web UI or REST API for these operations.

Proposed solution

Add three new tools to the security_advisories toolset:

• create_repository_security_advisory — create a new draft advisory (POST /repos/{owner}/{repo}/security-advisories)
• update_repository_security_advisory — update an existing advisory, including transitioning state to published (PATCH /repos/{owner}/{repo}/security-advisories/{ghsa_id})
• request_cve_for_repository_security_advisory — request a CVE ID from GitHub for a draft advisory (POST /repos/{owner}/{repo}/security-advisories/{ghsa_id}/cve)

This would allow security teams to manage the full advisory lifecycle — from draft creation through CVE assignment and publication — entirely within the MCP toolset, without leaving their workflow.

Example prompts or workflows (for tools/toolsets only)

1. Create a draft advisory: "Create a draft security advisory for a stored XSS vulnerability in our Core product affecting versions below 2.1.0, patched in 2.1.0."
2. Update an existing draft: "Update advisory GHSA-xxxx-xxxx-xxxx to add the CVSS vector string and change the severity to high."
3. Request a CVE ID: "Request a CVE ID from GitHub for advisory GHSA-xxxx-xxxx-xxxx."
4. Publish an advisory: "Publish advisory GHSA-xxxx-xxxx-xxxx."
5. End-to-end workflow: "Create a draft advisory for an OS command injection vulnerability in our product, request a CVE ID, and publish it once the CVE has been assigned."

Additional context

Relevant REST API documentation:

• [Create a repository security advisory](https://docs.github.com/en/rest/security-advisories/repository-advisories#create-a-repository-security-advisory)
• [Update a repository security advisory](https://docs.github.com/en/rest/security-advisories/repository-advisories#update-a-repository-security-advisory)
• [Request a CVE for a repository security advisory](https://docs.github.com/en/rest/security-advisories/repository-advisories#request-a-cve-for-a-repository-security-advisory)