Map user references to model handles

A small but high-impact detail: when the user attaches or references something, annotate the message so the model is handed the same handle it would use to act on it. If the user attaches a file, prefix the message with "the user attached: doc-2 (contract.pdf)". Now the model knows that "the contract the user just mentioned" is doc-2 and can read it directly. Without this mapping, the model has to guess which handle corresponds to the user's intent, a needless source of error.