fix: prevent AttributeError in _apply_settings when context has no agent0

[PaoloC68]

Summary

_apply_settings crashes with AttributeError: 'AgentContext' object has no attribute 'agent0' in contexts where agent0 has not been initialized yet (e.g. during early startup or in lightweight contexts created for settings application before the agent loop starts).

Root Cause

# Before
agent = ctx.agent0  # AttributeError if agent0 not set on ctx

AgentContext does not guarantee agent0 is set at the point _apply_settings is called. The attribute access raises AttributeError instead of gracefully handling the missing agent.

Fix

# After
agent = getattr(ctx, 'agent0', None)

One-line change. getattr with a default of None matches the existing null-check pattern already used downstream in the same function.

Impact

• Prevents a crash during settings application in contexts without an initialized agent
• No behavior change when agent0 is present
• Consistent with how other optional context attributes are accessed in the codebase