fix: align replacement-missing-policy prompt to spec

[rlippmann]

what changed

• Updated the case-7 clarify branch (use X instead of Y when Y is missing) to return the exact spec prompt: Did you mean to use "X" instead?
• Removed the previous implementation-specific multi-line diagnostic/confirm text for that case.
• Updated affected tests and fixture expectations that referenced the old wording.

why the change was needed

• The implementation and tests had drifted from the spec for clarify case 7.
• This change restores strict prompt contract alignment across engine behavior, checkpoint/fixture expectations, REPL rendering, transcript replay, and integration resume tests.