Ship Pi as a declarative Agent Extension for AgentGUI

[devRickyyy]

Goal

Split Pi integration from #978 and make Pi available as a selectable, runnable agent in Tutti's unified AgentGUI.

Required approach

Pi must be integrated through Tutti's Agent Extension Package plugin model:

• Publish Pi as an independent declarative Agent Extension, using pi-acp as its standard ACP runtime bridge.
• Tutti should register only the trusted extension source and dynamically consume the Agent Target, provider metadata, presentation, and runtime configuration supplied by the extension.
• Do not add a built-in local:pi target, closed Pi provider enum, static provider/catalog entry, provider-specific runtime adapter, or provider-specific CLI launcher.
• If the existing extension framework lacks a capability required by Pi, extend the framework generically rather than adding Pi-specific branches.

Done when

• Enabling the Pi extension makes Pi available in AgentGUI and supports a normal conversation through the shared Agent Extension and ACP paths.
• Disabling the extension removes Pi from new-agent selection.
• The Tutti main repository contains no statically compiled Pi provider integration.

Related feasibility spike: #1288.