docs: make the Agent skill install path discoverable

[jamiesun]

Summary

The Agent skill already lives at skills/sshx/SKILL.md, but the READMEs only mentioned sshx skill install inside the Go, Homebrew, and install-script notes. skills.sh and the skills CLI look for a clear install entry that discovers skills/*/SKILL.md.

• Add an Agent skill subsection at the top of Installation in README.md and README_CN.md.
• Document both install paths, each with one line of context:
  • npx skills add talkincode/sshx — skills.sh / skills CLI discovers skills/*/SKILL.md in this repository
  • sshx skill install — writes the skill embedded in an installed binary to ~/.agents/skills/sshx/SKILL.md
• Add a one-line pointer in the feature list next to the existing MCP item.

Existing Homebrew, Go, and install-script notes are unchanged.

Test plan

• Confirm README.md and README_CN.md both include npx skills add talkincode/sshx and sshx skill install in the new Installation subsection
• Confirm the feature-list pointer matches in both languages
• No code, CI, or unrelated docs changed