Development record

`.abcd/development/brief/04-surfaces/11-history.md:267-268` says the prompt-router entrypoint drains one entry and at most half a megabyte per prompt. `DrainBudget.exhausted` returns false when no entry has been attempted (`internal/core/history/staging.go:194`), so the first entry is redacted and stored whole at any size. The code comment gives the reason: otherwise a transcript over the bound would never leave the disk. The chapter states the bound with no such exception.