feat: multi-image per turn support

[yacosta738]

Summary

Support multiple images per turn in the image ingestion pipeline, as defined in the channel image ingestion strategy (GitHub #266).

Tasks

• Increase MAX_IMAGES_PER_TURN (configurable, default: 4)
• Update provider payloads to handle multiple images
• Update observability events for multi-image turns
• Add tests for multi-image scenarios

References

• Strategy spec: openspec/changes/2026-03-26-channel-image-ingestion-strategy/specs/channel-image-ingestion/spec.md
• Design: openspec/changes/2026-03-26-channel-image-ingestion-strategy/design.md (Future Considerations §1)
• GitHub parent issue: Define channel ingestion strategy for multimodal image input #266

Notes

• Current MVP enforces MAX_IMAGES_PER_TURN = 1
• Provider dispatch must be updated to pass &[StagedImage] as a multi-element slice
• This is a future enhancement — lower priority than Discord/Slack ingestion

[linear]

DALLAY-195