[desktop] Add /reload-plugins command for in-process plugin refresh (parity with CLI)

Title

[desktop] Add /reload-plugins command for in-process plugin refresh (parity with CLI)

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Body

Summary

The Claude Code CLI supports /reload-plugins, which refreshes skills, agents, hooks and MCP servers in-process without restarting the session. The Desktop App does not expose this command, so the only way to pick up plugin updates is to fully quit (Cmd+Q) and reopen the app — which loses the current session context.

Please bring /reload-plugins (or an equivalent menu item / keyboard shortcut) to the Desktop App.

Current behavior (Desktop App)

1. An external marketplace publishes new skills
2. Claude Desktop auto-updates the local cache under ~/.claude/plugins/cache/<marketplace>/<plugin>/<version>/ — new skill files are on disk
3. installed_plugins.json correctly points at the new version
4. The active Desktop session still shows the old skill list, because the in-memory registry was loaded at startup and is never refreshed
5. Workaround: Cmd+Q the whole app and reopen — destroys all open conversations' runtime state

Expected behavior

Running /reload-plugins (or Settings → Plugins → Reload, or Cmd+Shift+R style shortcut) should re-scan the local plugin cache and refresh the active registry without killing the session. Same semantics the CLI already has.

Why this matters

• Plugin marketplaces with automated releases push updates frequently; quitting the app every time is disruptive
• Plugin contributors iterate many times per day — hot reload is essential DX
• The feature already exists in the CLI, so parity is the minimal ask

Suggested implementation

• Wire the existing CLI /reload-plugins handler into the Desktop slash-command registry
• Optionally: surface a "Reload plugins" button in Settings → Plugins and a keyboard shortcut
• Respect scope: user-scope vs project-scope plugins should both refresh

Environment

• Claude Desktop App on macOS (Darwin 25.4.0)
• Marketplaces installed via /plugin marketplace add <git-url>

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Reporter: Victor Sá — Engineer @ Mercado Libre Brazil
Personal email: victoredo@gmail.com
Personal GitHub: @dev-vrsa