The MCP archive tool takes several repositories in one call, through the fleet operation, and its answer names the operation the undo tool reverts #2533

The rest of #1620. The home archives many at once with a 24-hour undo (#2512, 4c15a1c4) through the fleet archive verb (#2511). The MCP's archive tool in src/http/mcp.ts still takes one repo and calls setArchived directly, so an agent cannot do what the page does in one step.

Done looks like:

• [ ] The archive tool also accepts repos: [..] (names, at most 50). With repos, it runs the fleet archive operation (previewFleetIds while dryRun is true, the default; applyFleetIds when it is false) and answers each repository's outcome and the operation id. The single repo form keeps working unchanged.
• [ ] The tool's description says the existing undo tool (the one calling undoFleetOp) reverts it for 24 hours.
• [ ] The admin-token and never-an-agent rules stay as they are.
• [ ] Tests in test/archive-mcp.test.ts: a dry run over two changes nothing; applying archives both; the undo tool with the returned id wakes both; a non-admin's call is refused per row; the one-repo form still passes.

Files: src/http/mcp.ts and test/archive-mcp.test.ts. No new action code.