Add OpenCode Loop plugin

[CollieIsCute]

Summary

• Load @bybrawe/opencode-loop@0.5.1 through OpenCode's native npm plugin support
• Add the /loop command markdown stubs required for OpenCode command discovery
• Document the simplified OpenCode Loop setup

Verification

• diff -qr /tmp/opencode-loop-pack/package/commands /tmp/chezmoi-opencode-loop/home/dot_config/opencode/commands --exclude company-work.md
• chezmoi diff --source /tmp/chezmoi-opencode-loop/home ~/.config/opencode/opencode.json
• opencode debug config
• opencode debug startup