MCP `repo_settings`: change a repository's description, website and visibility, with the same rules as the API #4905

Intent: an owner tidies a repository from the harness; the existing repo tool only reads.

Done:

• [ ] src/http/mcp.ts has a repo_settings tool, placed right after repo, that calls what PATCH /api/v1/repos/:owner/:name call, for the same caller
• [ ] docs/MCP.md has its row beside repo
• [ ] test/mcp-reposet.test.ts: an admin changes the description and visibility and reads them back; a writer who is not admin is refused when the API refuses; an invalid visibility is refused with the API's error

Risks: test/mcp-size.test.ts holds tools/list under a 37,000-byte headroom, so keep the description to one short sentence. Other MCP tools are being added in parallel, so put this one exactly where named. Do not change the API routes: the tool calls the functions they call, with the same permissions.