fix: support OpenClaw 2026.7.2 plugin APIs

[vincentkoc]

Summary

• migrate prompt inspection from the retired before_agent_start hook to before_prompt_build
• use the current public plugin SDK entrypoints for plugin services and diagnostic events
• declare OpenClaw 2026.7.2 compatibility and document the required conversation-access permission
• add focused registration coverage

Verification

• npx vitest run (41 tests)
• SDK contract compile against openclaw@2026.7.2-beta.5
• git diff --check

Found through the OpenClaw HEAD compatibility canary in openclaw/crabpot.

This is intentionally separate from #8. That PR fixes service lifetime behavior and may need a small rebase after this API migration lands.