MCP `repo_labels` renames or recolours a label with `action: "update"`, as the labels page does #7497

Intent: an agent or a harness keeps a repository's labels tidy without the page. repo_labels lists, creates and removes labels. Only the labels page calls updateLabel (src/actions/repos.ts).

Done:

• [ ] repo_labels takes action: "update" with id, name and color (six hex digits), calls updateLabel and answers the label. not_found, invalid and taken are said in the page's words. It takes the same admin token as create.
• [ ] The tool's doc names the new action, and bun scripts/mcp-tools.ts regenerates the tool table. Keep the doc under the size test's ceilings (test/mcp-size.test.ts).
• [ ] A test (e.g. test/mcp-labels-update.test.ts) creates a label, renames and recolours it through the MCP route and sees the change on the list. It also checks that a taken name is refused and that a read-only token cannot update.

Files: src/http/mcp/tools/repo_labels.ts, docs/MCP.md, a new test.