Add UI Plugin Implementation using go-plugin for Hot-Reload/Hot-Deploy

[intel352]

Provide a UI plugin implementation using go-plugin, enabling UI hot-reload/hot-deploy for plugin-based admin or application UIs.

Tasks

• Implement go-plugin-based UI plugin loader for workflow engine.
• UI plugins can supply assets (static files), and optionally API handlers (routes).
• UI plugins can be redeployed/reloaded without restarting engine.
• Document conventions and API for UI plugin authors (asset directory, registration pattern, asset versioning).
• Update admin plugin navigation to support UI plugin hot-reload.

Acceptance

• UI plugins can be hot-reloaded, redeployed via go-plugin mechanism.
• Admin/application UI can be extended without core repo changes.

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Copilot Agent assigned (can be tackled in parallel).

Linked to broader UI refactor effort.