fix(agent-gui): preserve prompt asset restore metadata

[rv4no]

Summary

Preserve prompt asset identity, URI, upload state, and storagePolicy through Agent GUI input types, composer drafts, queued prompts, message projection, and history reads. storagePolicy is the sole durability contract; restore behavior is derived by the host adapter instead of carrying a separate restore-source field.

TSH uses this metadata to restore cloud-backed prompt images after runtime-local staged files disappear. This change only carries contract data; it does not change session, turn, goal, runtime-operation, upload, or recovery lifecycle semantics.

Verification

• 4 targeted spec files passed
• Agent GUI and activity-core typechecks passed
• Agent GUI build passed
• Agent degradation-boundary check passed
• pnpm check:changed -- --push-ready passed all 13 lanes, including pre-push

Checklist

• Agent lifecycle semantics unchanged; no host/conformance scenario required.
• I kept the change focused on one concern.
• Documentation not required; this propagates existing internal prompt asset contract fields.
• README/CONTRIBUTING language variants are unaffected.
• I ran the lowest meaningful local checks for the changed surface.
• Commit is signed off with DCO.

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[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no potential bugs to report.

View in Devin Review to see 2 additional findings.