fix: prevent agents from duplicating delegated subagent work

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

src/agents/hephaestus/gpt-5-4.ts

@@ -16,6 +16,7 @@ import {
   buildOracleSection,
   buildHardBlocksSection,
   buildAntiPatternsSection,
+  buildAntiDuplicationSection,
 } from "../dynamic-agent-prompt-builder";
 
 function buildTodoDisciplineSection(useTaskSystem: boolean): string {
@@ -115,7 +116,7 @@ When blocked: try a different approach → decompose the problem → challenge a
 - Run verification (lint, tests, build) WITHOUT asking
 - Make decisions. Course-correct only on CONCRETE failure
 - Note assumptions in final message, not as questions mid-work
-- Need context? Fire explore/librarian in background IMMEDIATELY — keep working while they search
+- Need context? Fire explore/librarian in background IMMEDIATELY — continue only with non-overlapping work while they search
 - User asks "did you do X?" and you didn't → Acknowledge briefly, DO X immediately
 - User asks a question implying work → Answer briefly, DO the implied work in the same turn
 - You wrote a plan in your response → EXECUTE the plan before ending turn — plans are starting lines, not finish lines
@@ -241,11 +242,13 @@ Prompt structure for each agent:
 - Fire 2-5 explore agents in parallel for any non-trivial codebase question
 - Parallelize independent file reads — don't read files one at a time
 - NEVER use \`run_in_background=false\` for explore/librarian
-- Continue your work immediately after launching background agents
+- Continue only with non-overlapping work after launching background agents
 - Collect results with \`background_output(task_id="...")\` when needed
 - BEFORE final answer, cancel DISPOSABLE tasks individually: \`background_cancel(taskId="bg_explore_xxx")\`, \`background_cancel(taskId="bg_librarian_xxx")\`
 - **NEVER use \`background_cancel(all=true)\`** — it kills tasks whose results you haven't collected yet
 
+${buildAntiDuplicationSection()}
+
 ### Search Stop Conditions
 
 STOP searching when you have enough context, the same information keeps appearing, 2 search iterations yielded nothing new, or a direct answer was found. Do not over-explore.