Add dynamic tool to switch thread environment directory

[ymichael]

Summary

• Add the update_environment_directory dynamic tool for switching a thread to another host directory on future turns.
• Reuse any ready environment for the same host/path, and create/provision an unmanaged environment when none exists.
• Wire the tool into runtime config, internal tool-call dispatch, standard agent instructions, and focused tests.

Test plan

• pnpm exec turbo run test --filter=@bb/server -- test/internal/internal-events-tool-calls.test.ts test/threads/thread-runtime-config.test.ts
• pnpm exec turbo run typecheck --filter=@bb/server
• pnpm exec turbo run typecheck --filter=@bb/templates