Genericize obsidian-sync skill for public use

[GoBeromsu]

Summary

The current obsidian-sync skill in the plugin repo is partially generic but still assumes a specific two-machine setup pattern. It should be fully genericized so any user with two machines can use it without modification.

Current issues

1. Description mentions "두 맥 동기화" — assumes Korean-speaking users with exactly two Macs
2. Troubleshooting section references Tailscale-specific networking without explaining it's optional
3. PM2 setup is documented but LaunchAgent (the newer approach) is not covered in the plugin version
4. Missing LaunchAgent template — the vault version has a working plist setup that should be ported as a generic template

Tasks

• Make description language-neutral and machine-count-agnostic
• Add LaunchAgent setup as an alternative to PM2 for macOS daemon management
• Clarify that Tailscale is one option for remote access, not a requirement
• Add placeholder-based configuration (already partially done with <REMOTE_HOST>, <VAULT_PATH>)
• Test with a fresh install to verify no assumptions leak through