Fix OpenClaw auto-capture dropping valid user text when recall context is prepended, and include the prior recall injection key fix in the same patch set.

Description

This PR consolidates two related OpenClaw plugin bug fixes in openclaw/index.ts:

1. Auto-recall injection fix
   before_agent_start now returns { prependContext: ... } (not systemContext) so recalled memories are actually injected into prompts.

2. Auto-capture preservation fix
   In agent_end, messages containing <relevant-memories>...</relevant-memories> are no longer skipped wholesale.
   Instead, injected recall context is stripped and the remaining real user/assistant text is captured if non-empty.

Root Cause #1: Hook return property mismatch (systemContext vs prependContext)
Impact #1: Auto-recall looked successful in logs but memories never reached the LLM

Root Cause #2: Marker-based hard skip in capture path
Impact #2: Valid user content was silently dropped whenever injected recall block was present

Fix #1: Rename systemContext → prependContext
Fix #2: Replace skip-on-marker with strip-and-keep behavior; skip only empty leftovers

Fixes #4037
Fixes #4063

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 and auto-capture in mem0 OpenClaw plugin
• Restarted openclaw gateway
• Started conversation that triggers memory recall
• Verified <relevant-memories> appears in effective prompt logs
• Verified capture no longer drops entire message when recall block is present
• Verified only injected block is removed and remaining user text is preserved/captured
• 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