feat: package Pi extensions as Bun workspaces

extensions/interactive-subagents/test/integration/agents/test-echo.md

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+---
+name: test-echo
+description: Integration test agent — completes simple file-writing tasks
+model: anthropic/claude-haiku-4-5
+tools: read, bash, write, edit
+spawning: false
+auto-exit: true
+disable-model-invocation: true
+---
+
+You are a test agent. Complete the task given to you immediately. Be direct and concise.
+When asked to write content to a file, do it right away using the bash tool.
+Do not ask questions. Do not explain. Just execute the task.

extensions/interactive-subagents/test/integration/agents/test-fork.md

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+---
+name: test-fork
+description: Integration test agent — fork-mode child that inherits the parent session
+model: anthropic/claude-haiku-4-5
+tools: read, bash
+session-mode: fork
+auto-exit: true
+disable-model-invocation: true
+---
+
+You are a test agent. Complete the task given to you immediately. Be direct and concise.
+When asked to write content to a file, do it right away using the bash tool.
+Do not ask questions. Do not explain. Just execute the task.

extensions/interactive-subagents/test/integration/agents/test-ping.md

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+---
+name: test-ping
+description: Integration test agent — calls caller_ping instead of completing task
+model: anthropic/claude-haiku-4-5
+tools: read, bash
+spawning: false
+disable-model-invocation: true
+---
+
+You are a test agent. When given ANY task, you must call the caller_ping tool with the message set to "PING: " followed by the task text you received.
+Do NOT complete the task yourself. Do NOT use any other tools. ONLY call caller_ping.