Mervin's board: cross-file checks, and replay the board with pretend agent teams #1484

Follow-ups from #1479 (scripts/mervin-board.ts):

• A "certain" tier covers one file's text only. Add board-wide checks for clashes that span files: migrations with the same stamp (codeSplitter: protect main from crew and agents #1432 vs main), two PRs adding the same export or route, and so on.
• Once codeSplitter: the borg engine, Mervin rolling through public histories with pretend agent teams #1468 lands, feed board.json into borg-agents.ts so pretend agent teams replay the board's stages concurrently against one file Durable Object, and compare the result with the sandbox tree.
• --check tests only the final tree. Add a per-stage check (one install per stage, opt-in) so a failure points at the stage that caused it.

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