Fix packaged provider bridge runtime startup

[ymichael]

Summary

• Launch runtime-managed Node bridge providers with an explicit Node runtime instead of relying on node on GUI app PATH
• Pass ELECTRON_RUN_AS_NODE=1 only to Electron-launched bridge processes
• Cover Claude Code, Pi, and ACP bridge adapters on current main
• Strip bridge-only Electron runtime env before launching downstream Claude/ACP agent subprocesses

Repro

• Confirmed spawn node fails with ENOENT when PATH is empty
• Confirmed process.execPath still launches successfully with empty PATH
• Added a regression test that clears PATH and starts a runtime-managed bridge provider

Validation

• pnpm exec turbo run typecheck --filter=@bb/agent-runtime
• pnpm exec turbo run test --filter=@bb/agent-runtime --force

Replaces #220.