Agents subnav: interactive + pipeline tabs

Add the agents subnav with "interactive" and "pipeline" tabs, similar to how fund and tasks have subnavs.

Subnav Tabs

1. interactive — /agents — The new interactive agents dashboard (default) 2. pipeline — /agents/pipeline — The existing automated pipeline dashboard (re-mounted under /agents)

Implementation

• Add agentsSubnav :: Text -> Html () to Omni.Web.Style (like fundSubnav, tasksSubnav)
• Use sharedShellWithSubnav in both the interactive agents page and the pipeline page
• Update Omni/Web/Core.hs routing:
• /agents → new agents web app
• /agents/pipeline/* → existing Pipeline.Web app (strip prefix and forward)
• /pipeline/* → redirect to /agents/pipeline/* (backward compat, optional)

Notes

• The pipeline web app already accepts a basePath parameter, so remounting it under /agents/pipeline should work by passing the new base path.
• The pipeline page should show the agents subnav with "pipeline" tab active.