Nav: Replace 'pipeline' with 'agents' in top nav

Update the shared nav in Omni/Web/Style.hs:

• Replace "pipeline" nav link with "agents" (href /agents)
• Update Omni/Web/Core.hs to route /agents/* to a new Omni.Agents.Web module
• Keep /pipeline/* routing working (it will be a subnav under agents later)

Files to modify:

• Omni/Web/Style.hs - sharedNav function
• Omni/Web/Core.hs - add agents route

This is a small, surgical change. The agents route can initially just redirect to pipeline or show a placeholder.