#4659 stagecraft DESIGN.md: say the stored-as-received check catches only a redacted secret, and --literal-inputs is what stops evaluation

Extension: @swamp/stagecraft (swamp-extensions repo). Follow-up from the swamp-club#4596 review.

DESIGN.md's bullet 'A payload is stored as received, or not recorded' is literally true but reads as a guard against expression evaluation in general. The read-back check only refuses when swamp changes a payload on write, which happens when it redacts a vault secret resolved during the run. Without --literal-inputs, any other expression in a payload (for example an inputs or data reference) is evaluated before the method runs, stored unchanged, and recorded with no error.

Expected: one sentence in DESIGN.md (and REFERENCE.md where it describes writes) saying the read-back catches a redacted secret only, and that --literal-inputs, which the skill passes on every write carrying text, is what keeps text from being evaluated.