feat(plugin): add update command for third-party plugins

[saulob]

This PR introduces a new CLI command to update installed third-party plugins

New command
inkypi plugin update <plugin_id>

Behavior

• Reads the repository URL from plugin-info.json of the installed plugin
• Downloads the latest version of the plugin
• Replaces the installed plugin files
• Installs requirements if requirements.txt is present
• Restarts the service after update

Why

Currently, the CLI only provides install, uninstall and list commands for third-party plugins. Updating a plugin requires re-running the install command manually. This PR adds a dedicated update command, making the workflow clearer and safer

Notes

• The update process validates the new version before replacing the existing plugin, avoiding accidental deletion on failed downloads
• The implementation follows the same approach as install, reusing the repository source stored in plugin metadata

Manual testing

• Install a plugin using inkypi plugin install
• Modify the plugin repository (e.g. change a label or text)
• Run inkypi plugin update <plugin_id>
• Confirm that changes are applied
• Confirm that dependencies are installed if requirements.txt exists
• Confirm that service restarts correctly

[Copilot]

Pull request overview

Adds a dedicated inkypi plugin update <plugin_id> command to the existing plugin CLI to update an installed third-party plugin from its recorded repository, reinstall dependencies when needed, and restart the service.

Changes:

• Extended CLI help output to include plugin update.
• Implemented update_plugin() to clone the plugin repo into a temp directory, stage validated files, replace the installed plugin, optionally install requirements.txt, and restart the service.
• Wired the new update subcommand into the command router.

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