fix(agents): prevent orchestrator from rushing ahead of background agents

Tighten Background Result Collection instructions so the model waits
for explore/librarian results instead of duplicating their work and
delivering premature answers.

- Remove 'Continue working immediately' which models interpreted as
  'do ALL work yourself, ignore agents'
- Clarify step 2: only do DIFFERENT independent work while waiting
- Add explicit step 3: end response when no other work remains
- Add 'not for files you already know' to explore section header

Fixes #2124, fixes #1967

src/agents/dynamic-agent-prompt-builder.ts

@@ -116,7 +116,7 @@ export function buildExploreSection(agents: AvailableAgent[]): string {
 
   return `### Explore Agent = Contextual Grep
 
-Use it as a **peer tool**, not a fallback. Fire liberally.
+Use it as a **peer tool**, not a fallback. Fire liberally for discovery, not for files you already know.
 
 **Use Direct Tools when:**
 ${avoidWhen.map((w) => `- ${w}`).join("\n")}