Enhancement: Show all available backends in status command

[Cannon07]

Summary

The :CodePreviewStatus command (currently :ClaudePreviewStatus) only checks for Claude Code hooks. It should show the status of all available backends with install hints, making the plugin self-documenting for new users.

Current behavior

claude-preview.nvim status
────────────────────────────────────
Neovim socket : /tmp/nvim.sock
Hooks         : not installed
jq            : found
Diff tab      : closed

Only checks for Claude Code hooks in .claude/settings.local.json. No mention of OpenCode.

Desired behavior

code-preview.nvim status
────────────────────────────────────
Neovim socket : /tmp/nvim.sock
jq            : found
Diff tab      : closed

Backends:
  Claude Code : not installed  →  :CodePreviewInstallClaudeCodeHooks
  OpenCode    : not installed  →  :CodePreviewInstallOpenCodeHooks

• Show all available backends, not just installed ones
• Each backend shows its install status and the command to install/uninstall
• Helps new users discover what backends are available and how to set them up

Detection logic

• Claude Code: check for hook entries in .claude/settings.local.json (look for the hook marker string)
• OpenCode: check for plugin files in .opencode/plugins/ (e.g., index.ts)

Notes

• Blocked by Rename plugin from claude-preview to code-preview (backend-agnostic) #13 (rename refactor) — command names will change
• Should be a small, self-contained change to M.status() in init.lua

[Cannon07]

Implemented in #38 — :CodePreviewStatus now shows all backend install status with hints.