feat(harness): expose native Pi runtime

[jonathanlab]

This cleans up the entry points towards the harness, which were getting quite muddled

It also introduces a way to spawn pi in RPC mode in @posthog/agent via rpc-host.ts, and a way to interact with the RPC host in a typed manner via rpc-client.ts

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[jonathanlab]

• feat: hook up pi runtime to ChatThread #3474
• feat(harness): integrate pi runtime into desktop app #3469
• feat(harness): expose native Pi runtime #3391 👈 (View in Graphite)
• main

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[github-actions]

React Doctor found no issues in the changed files. 🎉

_{Reviewed by React Doctor for commit 0e925bf.}

[greptile-apps]

Security Review

The new runtime can reuse credentials from the initial agent directory when Pi recreates a runtime with a different directory. That can cross account or region boundaries in embedded runtime flows.

_{Reviews (1): Last reviewed commit: "feat(harness): expose native Pi runtime" | Re-trigger Greptile}