[codex] Add subagent context hygiene guidance

[morganmcg1]

Summary

• Add a concise advisor context-hygiene contract: delegate bulky evidence scans, keep subagent trees one level deep, and return compact sourced summaries.
• Reinforce the doctrine in advisor/student runtime prompts, heartbeat prompts, launch-wide extra instructions, assignment PR handoffs, result submission expectations, and researcher-agent output guidance.
• Update bootstrap target templates so future target packages can encode durable research memory and delegation expectations from the start.

Validation

• python -m py_compile k8s/launch.py
• bash -n k8s/entrypoint-advisor.sh
• bash -n k8s/entrypoint-student.sh

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