feat(tui): ingest image attachments through Runtime Host

[me2seeks]

Summary

Add a self-contained TUI image-attachment ingestion path that keeps a user-facing marker such as [image 1] while carrying the exact durable Session Artifact reference into model-facing message content.

This is split from #4079 / #4080 because the current TUI has no attachment ingestion or staging authority: its submit path carries text only, while Runtime Host accepts only canonical session_file attachments already committed to the Session ArtifactStore.

Required flow

1. Accept an explicit image attachment action in TUI without blocking Send.
2. Ingest bytes through the existing Runtime Host artifact.ingest authority.
3. Keep image bytes and filesystem paths out of editor text; show an atomic marker such as [image 1].
4. Submit the exact AttachmentRef alongside the visible label.
5. Preserve attachment identity through current-turn, steering, queued-successor, retry/retraction, and recovery paths.
6. Define deletion, failed-ingest, abandoned-draft, and orphan-artifact cleanup behavior.

Acceptance criteria

• A TUI image marker serializes to the exact committed Session Artifact reference.
• Non-vision models receive the attachment name, MIME type, and Read reference without provider-native image bytes.
• Vision models retain the existing native-image path.
• Marker editing cannot silently bind a different image or leak a local path.
• Tests cover upload, marker-to-reference mapping, deletion, submission, queue/retraction, and recovery.
• Local and remote Runtime Host ownership boundaries remain explicit.

Non-goals

• Do not create a second attachment store or Surface-owned durable authority.
• Do not infer OCR capability or automatically switch models.
• Do not expose temporary image paths in prompts or transcripts.

Refs #4079
Refs #4080

AI assistance disclosure: Drafted and published with Codex at the author's direction.

[abhinav-phi]

I’d like to take this issue up. I can work on the TUI image-attachment ingestion flow, including the Runtime Host artifact flow, marker-to-AttachmentRef mapping, attachment lifecycle/recovery handling, and the required test coverage.

Could you please assign this to me?

/assign