Record the source state a memory or page revision was checked against #301

Part of #216's evidence metadata. A memory or documentation page describes current behaviour; what matters later is which state of the code it was verified against, not every commit that ever touched it. Commit lists stay on tasks (#299).

Work

• Optional verifiedAgainst per memory and page revision: full commit SHA plus the repository as it was when recorded, set together with #216's verification time. Editing prose without new verification carries no value forward as fresh.
• memory_store and page_store accept it; item_get and recall return it where #216 shows freshness.
• Web: shown on the detail and history views, linked to the host only for a verified project remote.

Acceptance

• A new revision without verifiedAgainst does not inherit the previous one as current verification; history still shows the old value.
• Same SHA validation and visibility rules as task commit links (#299).
• Covered by #98's evaluation cases where applicable.