agent: add NodeBody::Image for Qwen3-VL vision input

Images are rendered as `<|vision_start|>` + N × `<|image_pad|>` +
`<|vision_end|>` where N is computed from the image dimensions using
Qwen3-VL's smart_resize rules (patch_size=16, merge_size=2, min=64K,
max=16M pixels). The token count matches what vLLM will produce at
request time, so budget accounting stays accurate.

Bytes are stored inline on the leaf and base64-encoded in the JSON
form. Token IDs are hand-assembled instead of re-running the tokenizer
on a potentially-huge placeholder string.

Follow-ups: view_image tool rewrite, multi_modal_data on the vLLM
request, API-layer plumbing from leaf bytes to request body.

Co-Authored-By: Proof of Concept <poc@bcachefs.org>

src/user/chat.rs

@@ -486,6 +486,11 @@ impl InteractScreen {
                         if t.is_empty() { vec![] }
                         else { vec![(PaneTarget::ToolResult, text, Marker::None)] }
                     }
+                    NodeBody::Image { orig_height, orig_width, .. } => {
+                        vec![(PaneTarget::Conversation,
+                              format!("[image {}x{}]", orig_width, orig_height),
+                              Marker::None)]
+                    }
                 }
             }
             AstNode::Branch { role, children, .. } => {