When a user attached a binary document (PDF, DOCX, XLSX, …) in chat, the
context note prepended to the turn said "Ask the user what they'd like you to
do with it." That steered the model into asking the user to paste the
contents rather than extracting the text it is fully capable of reading — so
attached PDFs/DOCX appeared "unreadable" to the agent.
Rewrite the binary-document note to tell the agent the file is a non-text
format saved at the given path and to extract its text itself (e.g. via the
terminal tool or the ocr-and-documents skill) before answering. Text
documents (whose content is already inlined by the platform adapter) keep
their existing note. The note construction is pulled into a small
`_build_document_context_note` helper so it is unit-testable.
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