Change thread open to resolve workspace paths

[SawyerHood]

Summary

• Change bb thread open to accept a single path argument and resolve it from process.cwd() unless absolute.
• Resolve the matching non-archived thread by longest workspace-path prefix and print its BB thread URL.
• Update CLI tests, command help expectations, and bundled guidance/docs for the path-only behavior.

Validation

• pnpm exec turbo run typecheck --filter=@bb/templates
• pnpm exec turbo run test --filter=@bb/cli
• pnpm exec turbo run typecheck --filter=@bb/cli
• pnpm exec turbo run build --filter=@bb/cli
• pnpm exec turbo run build --filter=@bb/server