Fix editor thread storage host routing

[bb-slop-cop]

🚨 SLOP COP 🚨 · new-issue-autopilot

Human comments

What was wrong

The editor resolved thread-storage roots from server-local configuration and omitted the owning host ID. Remote-thread read, tree, and save requests consequently targeted the server. Reproduction report.

What changed

Resolve the existing thread storage location through the SDK and forward its root and host ID to file operations. Remove the local-root helper and add three routing regression tests. Total: 112 changed text lines across two files (103 additions, 9 deletions).

How you verified

• Trusted main and a second clean checkout: three routing tests failed; 34 existing tests passed.
• With the fix: corepack pnpm exec turbo run test typecheck --filter=bb-plugin-monaco-editor passed, including all 37 tests.
• Full base Turbo build passed; git diff --check passed.
• Tests exercise the real plugin RPC handlers with SDK test doubles. Live two-machine browser behavior and physical writes were not exercised.

Fixes #3604

AGENT GENERATED