$AGENT_HOME resolves to the workspace directory, not the instructions
directory where sibling files (HEARTBEAT.md, SOUL.md, TOOLS.md) live.
This caused ~25% of agent runs to fail. Relative paths align with the
adapter's injected directive to resolve from the instructions directory.
Closes#2530
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>