Tell the model that [REDACTED] was inserted by marginal #178

Context

On #176, a doc explained redaction with the example "https://user:s3cr3t@host becomes https://[REDACTED]@host". marginal redacted the example's user:s3cr3t@ before the model saw it. So the model read "https://[REDACTED]@host becomes https://[REDACTED]@host" and flagged the example as a no-op (Medium, 85%), a false positive created by marginal itself.

The same will happen to any docs, tests or fixtures that show a secret-shaped example: the model sees [REDACTED] and has no way to know that marginal put it there.

Behavior to implement

When redaction changed anything in the prompt, tell the model so: [REDACTED] marks a value marginal removed before review, the original text had a real-looking value there, and the placeholder itself must never be flagged or quoted as the code's content.

Acceptance criteria

• With a redacted value in the diff, the prompt contains the note; without one, the prompt is unchanged.
• An eval case: a diff documenting a redaction example gets no "no-op" or "placeholder" finding.