fix(agents): declare multimodal-looker tool allowlist in prompt to prevent death loop on small VL models (fixes #4116)

The multimodal-looker prompt described what to do but never told the model which tools are available. Smaller VL models (e.g. Qwen3-VL-8B) would try to call non-existent tools and enter an infinite loop emitting:

  Model tried to call unavailable tool 'invalid'. Available tools: call_omo_agent, read.

Add a single sentence at the top of the prompt that explicitly enumerates the only allowed tools ('read' and 'call_omo_agent') and forbids calling any other tool. This matches the runtime allowlist enforced by createAgentToolAllowlist(["read"]).

Regression test asserts the prompt contains the available-tools enumeration so future prompt rewrites don't regress.

src/agents/multimodal-looker.ts

@@ -23,6 +23,8 @@ export function createMultimodalLookerAgent(model: string): AgentConfig {
     ...restrictions,
     prompt: `You interpret media files that cannot be read as plain text.
 
+Your only available tools are 'read' and 'call_omo_agent'. Always use 'read' to load the file first, then analyze the returned content. Never attempt to call any other tool.
+
 Your job: examine the attached file and extract ONLY what was requested.
 
 When to use you: