The agent seat can do what the other two can: edit, delete and add a column over MCP #126

Pushed. Here's the pull description:

Adds three tools to src/mcp.ts so an agent can finish work on a board, not just add to it: edit_card (board_id, card_id, title), delete_card (board_id, card_id), and add_column (board_id, name). Each validates exactly as its counterpart in src/routes.ts does — the same ENTITY_ID shape, the same trimmed/length-capped title (200) or column name (60), the same not_found/invalid/full error shapes with a way out (e.g. "call list_cards for this board's card ids") — and publishes its bus event (card.edited, card.deleted, column.added) with the caller's identity, so the change shows up on every open page within a second, same as a person's. add_column past MAX_COLUMNS_PER_BOARD fails with the number in the message.

tools/list now returns eight tools in total; the README's Agents bullet lists all eight and calls out the call order a model needs.

Left out, as the issue asks: undo (board-wide and destructive — an agent reversing a person's change is its own decision), and deleting a column (cascades every card in it). A deleted card stays undoable from the page itself, which is the mitigation for the first destructive tool an agent now has.

New tests in src/server.test.ts: edit_card/delete_card change the board and are seen on GET /b/:id/events within a second; a bad card id is refused without touching the board; add_column publishes its event and is refused once the board hits its column cap, leaving the board unchanged. bun test passes (200 pass, 0 fail) — note node_modules wasn't installed in this clone, so bun install was needed first; that's environment, not code.

🤖 Generated with Claude Code

Closes #68