A test pins MCP `board_delete`: a person admin removes a board, an unknown slug is `not_found`, an agent is refused #7059

The MCP tool board_delete (src/http/mcp/tools/board_delete.ts) has no test of its own; only the tool list names it.

Done:

• [ ] A new test calls the tool through /mcp. A forge admin who is a person deletes a board, and GET /api/v1/boards/:slug is then 404.
• [ ] An unknown slug answers not_found with "no such board".
• [ ] An agent admin and a non-admin are refused, and the board stays.

Files: test/mcp-board-delete.test.ts

If the test finds the behaviour wrong, fix the tool in the same pull and say so.

If the check fails in a test this pull does not touch, name it in the pull rather than widening the change.

Before pushing: bun x tsc --noEmit and the tests this names.