fix: check top-level instruction field for agent .md file instructions

[bkrabach]

Summary

Companion fix to amplifier-foundation PR that ensures agent instructions from .md files are properly used when spawning agent sessions.

Problem

The session spawner was only checking agent_config.get("system", {}).get("instruction") for the agent's system instruction. However, when agent instructions come from the markdown body of agent .md files, they are now exposed at the top level as instruction (after the foundation fix).

Solution

Updated session_spawner.py to check for instruction at the top level first, then fall back to the nested system.instruction path:

system_instruction = agent_config.get("instruction") or agent_config.get(
    "system", {}
).get("instruction")

Related

• Depends on: fix: include agent instructions from .md files in spawned sessions amplifier-foundation#32 (foundation fix to include instruction in agent metadata)