Add agents and customise subagent extension to pass reasoning effort and allow overriding which model is used.

.pi/prompts/implement.md

@@ -0,0 +1,10 @@
+---
+description: Full implementation workflow - scout gathers context, planner creates plan, worker implements
+---
+Use the subagent tool with the chain parameter to execute this workflow:
+
+1. First, use the "scout" agent to find all code relevant to: $@
+2. Then, use the "planner" agent to create an implementation plan for "$@" using the context from the previous step (use {previous} placeholder)
+3. Finally, use the "worker" agent to implement the plan from the previous step (use {previous} placeholder)
+
+Execute this as a chain, passing output between steps via {previous}.