fix: keep ask_user images and pin them across compaction

[justrach]

Closes #580. Closes #581.

What happened

Two related failures in the same session (choosing-an-editable-notes-platform on 0.0.267):

1. ask_user image uploads reach the model only as [Image] placeholders #580 — ask_user accepted nine screenshots. The tool result delivered to the model was nine literal [Image] placeholders. Pixels never left the readline buffer.
2. Compaction can discard images from the unresolved current user prompt #581 — The same turn compacted 255k → 16k tokens. The text of the current prompt survived in the summary; its image blocks did not.

What we do now

#580. Every successful paste/drop/@[path] appends to a 16-slot queue (the last image still fills pending_image for history nav). ask_user still returns text — Responses/OpenAI tool output is a string — and vision_queue.flushPending then appends one follow-up user message with the same vision blocks a normal prompt already builds (ADR 0012). If the reply has [Image] markers and the queue is empty, the tool result names path / paste-text / next-prompt fallbacks instead of implying the pixels arrived.

#581. recentContextStart and emergencyCutIndex pin the opening user of an unresolved turn (and any first-turn image prompt). A HungRequest retry sees the same cut. If that pin is the whole history, compact returns error.ActivePromptPinned instead of summarizing attachments to text. Traces emit compact_cut with the boundary and preserved image count, not payloads. Child index-0 mandates stay on the pinChildTask path.

Tests

• Two staged images become two input_image blocks on the next request.
• Unresolved image + fat tool tail stays at the opening user; a second recentContextStart call does not move the cut.
• Emergency trim refuses to drop that prompt.
• First-turn images refuse a summarize-all cut.

Out of scope

ACP / PR #573 is untouched. This branch is from main.

  

[justrach]

Closing as landed. ask_user images ride a follow-up user message after the text tool result (ADR 0015); vision_queue.flushPending and ActivePromptPinned are on tip.