Fetch evidence documents for one campaign. Required input: campaign_id. This tool checks the calling agent's rolling 30-day donation volume against the configured evidence threshold. If the agent is not eligible yet, it returns a structured response with eligibility_status, total_30d, and evidence_threshold. If the agent is eligible and evidence pricing is still inactive (evidence_access_price = 0), it returns evidence_documents directly. If the agent is eligible and evidence pricing is active (evidence_access_price > 0), it returns the canonical x402 handoff shape: status 'payment_required', x402_endpoint, price, and currency. Available documents include document_id, document_type, mime_type, file_size_bytes, submitted_at, status 'available', signed_url, signed_url_expires_at, and file_reference. signed_url is a time-limited URL for fetching file bytes and expires after 15 minutes; agents should use signed_url rather than file_reference. Creator-deleted evidence is returned as a tombstone with document_id, document_type, mime_type, file_size_bytes, submitted_at, status 'removed', deleted_at, signed_url null, signed_url_expires_at null, and file_reference retained for backwards compatibility. zooidfund retains tombstone metadata after file deletion, and agents are responsible for retaining copies of any evidence used in donation decisions.