docs: add Hermes to README and note Overview agent visibility

[RealZST]

Summary

Docs-only update reflecting Hermes agent support (#74) and clarifying how the Overview lists agents.

• 9 agents — bump the count from 8 → 9 and add Hermes to the supported-agents prose, the capability table (Skills / MCP / Plugins / Hooks / Agent-first CLI all ✓), and the hk status example.
• Overview visibility note — add an italic caption under the agents banner explaining that every supported agent shows on the Overview by default, installed or not, and how to hide the ones you don't use via Settings → Agent Paths (click the Enabled toggle to switch it off).
• Mirrored across README.md and README.zh-CN.md (zh uses the in-app term 「概览」 / 「智能体路径」).

Hermes row verified against crates/hk-core/src/adapter/hermes.rs — it implements skill_dirs, MCP, hooks (HookFormat::HermesYaml), and plugins.

Test plan

• Rendered both READMEs locally; caption is centered under the banner, table row aligns.
• No code changes — documentation only.

🤖 Generated with Claude Code