bug(rewind): report and preserve recoverable prompt attachments #929

Expected behavior

Return structured attachment recovery status and loss reports; retain selectable supported image-only boundaries and explicitly describe unavailable/unsupported attachments without substituting changed filesystem bytes.

Source finding and impact

Codex/OpenCode draft adapters use plainDraft, which selects text and returns no images. promptsFromSnapshot omits entries whose reconstructed text and images are empty. Image-only prompts and unsupported attachment loss are therefore not represented by the draft contract.

Evidence and environment

Inspected source at 552914f5610518458f944fa1bdaf63f243685bd1 on 2026-09-12 in the local macOS workspace. This finding has not yet been reproduced through a running application or the package integration suite.

• src/main/providerSwitch/transcriptEngine.ts
• src/main/providerSwitch/rewindSession.ts

Regression and acceptance

Real decoder fixtures for text+image, image-only, missing attachment and changed path content must reach picker and draft reconstruction with exact restored or unavailable evidence. Preserve the existing rule that a rewind needs a resumable retained prefix.

The resolving PR must protect the observable contract and document any remaining uncertainty. This planning pass makes no runtime changes.

Refs #918; planning finding F16.