feat(agent): vision-v1 tier + vision sub-agent for image understanding (#3699)

Co-authored-by: Steven Enamakel <enamakel@tinyhumans.ai>

docs/TEST-COVERAGE-MATRIX.md

@@ -279,6 +279,7 @@ End-to-end coverage of the agent harness via the web-chat RPC surface against an
 | 6.3.1 | Steer a running sub-agent    | RU    | `src/openhuman/agent_orchestration/running_subagents.rs`, `src/openhuman/agent_orchestration/tools/steer_subagent.rs` | ✅     | `steer_subagent` injects a steer/collect message into a running async sub-agent's run-queue; registry enforces parent ownership + terminal guard.  |
 | 6.3.2 | Wait for a sub-agent result  | RU    | `src/openhuman/agent_orchestration/running_subagents.rs`, `src/openhuman/agent_orchestration/tools/wait_subagent.rs`  | ✅     | `wait_subagent` blocks on the completion `watch` with a timeout; prunes terminal entries, leaves entries intact on timeout.                        |
 | 6.3.3 | Steer lands in child history | RU    | `src/openhuman/agent/harness/subagent_runner/ops_tests.rs::run_queue_steer_lands_in_subagent_history`                 | ✅     | End-to-end: a queued steer is drained by the child `run_turn_engine` and appears as a `[User steering message]` user turn in the provider request. |
+| 6.3.4 | Vision sub-agent reads attached images | RU | `src/openhuman/agent_registry/agents/loader.rs::vision_agent_loads_on_vision_hint`, `src/openhuman/inference/provider/factory_tests.rs::vision_tier_is_vision_capable`, `src/openhuman/agent/harness/engine/core.rs::gate_tests`, `src/openhuman/agent/multimodal_tests.rs::extract_image_placeholders_pulls_att_tokens_in_order` | ✅ | Orchestrator (non-vision `chat-v1`) keeps the image as a placeholder, delegates to `vision_agent` on the `vision-v1` tier, which rehydrates the on-disk attachment and reads it. Engine gate prefers per-tier `current_model_vision`; turn placeholders forwarded into the sub-agent prompt. |
 
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