Track observed environment branches

[ymichael]

Summary

• record daemon-observed environment branch/default-branch metadata after successful workspace status checks
• add DB helper coverage for updating and clearing environment branch metadata
• add a scoped plan for making New worktree default to detached HEAD with an explicit branch-backed option

Tests

• pnpm exec turbo run test --filter=@bb/db --force
• pnpm exec turbo run test --filter=@bb/server --force
• pnpm exec turbo run typecheck --filter=@bb/db --filter=@bb/server
• git diff --check