agent: dedupe transcript scanning in opencode and copilot-cli

opencode's ExtractModifiedFilesFromOffset and ExtractModifiedFiles repeated the assistant-message/tool-part file collection loop — extract modifiedFilesFromMessages and call it from both. copilot-cli's summary and model extraction repeated the scan-newest-first/unmarshal/return pattern three times — extract lastEventField[T] and express the model-change-then-tool-telemetry fallback as two calls. Covered by the existing transcript extraction tests in both packages.