Change systemContext to prependContext in before_agent_start hook to match OpenClaw's expected property name. Memories were being fetched but never injected into prompts.

Description

Fixes a critical bug where auto-recall memory injection was silently broken. The before_agent_start hook returned { systemContext: ... } but OpenClaw's hook runner only checks for { prependContext: ... }. This caused memories to be fetched and logged but never actually injected into conversation prompts.

Root Cause: Property name mismatch in hook return object
Impact: Auto-recall appears to work (logs show injection) but memories never reach the LLM
Fix: One-line property rename from systemContext to prependContext

Fixes #4037

Type of change

• Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

Manual testing steps performed:

• Set OPENCLAW_CACHE_TRACE and OPENCLAW_ANTHROPIC_PAYLOAD_LOG to true
• Enabled auto-recall in mem0 OpenClaw plugin
• Restarted openclaw gateway
• Started conversation triggering memory recall
• Verified now appear in .openclaw/logs
• Verified system prompt (agent.md, soul.md) remains unaffected

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules
• I have checked my code and corrected any misspellings