Fixes#202
## Summary
- keep the default `root` worktree directory pointed at the folder the
user opened
- continue using the git repo root only for git/worktree discovery
- add a targeted regression test for opening a repo subfolder as the
workspace
## Why
When a workspace is opened from a subfolder inside a git repo, CodeNomad
currently maps the `root` worktree to the repo root. That causes proxied
OpenCode requests to run with the repo root directory and miss an
`opencode.json` that lives in the selected subfolder.
## Validation
- inspected the attached `config-issue.zip` from #202
- confirmed `resolveRepoRoot(proj-1)` still returns the git root while
`listWorktrees()` now returns `root.directory = proj-1`
- `npx tsx --test
"packages/server/src/workspaces/__tests__/git-worktrees.test.ts"`
- `npm run typecheck --workspace @neuralnomads/codenomad`
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