chore(quality): ratchet open-sse typecheck baseline to zero (#12418)

The current release dependencies already resolve the ChatGPT Web vendor diagnostics the allowance covered, so the stale open-sse typecheck allowance is removed and any future diagnostic becomes a blocking regression again. No vendor source, package manifest, checker script or runtime behaviour changes.

Validated in a combined worktree with the batch's ready set boarded onto the current tip: parse sweep clean on every changed TypeScript file, typecheck:core clean, check:dashboard-typecheck OK (207 pre-existing errors, all within baseline), check:cycles OK, check-file-size OK, 203/205 focused node tests and 94/94 vitest — the two failures belong to #12427, which is held back.