A test pins MCP `board_write`: a person admin saves a board, an agent and a non-admin are refused, and a bad board is not saved #7058

The MCP tool board_write (src/http/mcp/tools/board_write.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 writes a board by slug, and GET /api/v1/boards/:slug then answers it.
• [ ] An agent admin and a non-admin are refused with boardRefused's words, and nothing is written.
• [ ] A board the store refuses (an unknown source, a malformed query) answers refused with the reason, and the old board is unchanged.

Files: test/mcp-board-write.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.