[Feature]: "Add as project" in File Explorer context menu for nested Git repos #31028

Problem or use case

I work in "meta-repo" workspaces: one Git repo that pulls a few dozen independent service repos into subfolders (synced by scripts, gitignored in the parent). I open the parent as my Orca project so agents see everything in one place.

For a feature or bugfix I usually need one or two of those nested repos as real Orca projects for a few days (worktrees, branch, git status, agent sessions scoped to that repo). Right now that means opening the add-project dialog and browsing to parent/some/nested/repo every time, or running orca repo add --path ... from a terminal. The repo is already visible in the File Explorer, so the detour feels unnecessary.

Proposed solution

In the File Explorer context menu, when the selected folder is a Git repo root (contains .git), show an extra entry:

• Add as project — registers the folder like orca repo add --path <folder> does
• if the parent project is in a project group, put the new project into the same group (or offer "Add to group ▸")

Optional: mark projects added this way so they are easy to find and remove again later, e.g. a small "nested in " hint or a filter.

Removing them in bulk afterwards is covered by #24542.

Alternatives or additional context

• Workaround today: orca repo add --path <folder> from a terminal, then "Move to group" by hand.
• Related, but about showing git status for nested repos rather than registering them as projects: [Feature]: Multi-repo support — show Git status/tree for nested repos under the opened workspace folder #13989, [Feature]: Support nested Git repositories within a single added repo/workspace #23385
• Related: [Feature]: Watch a parent folder and offer to add new repositories as projects #20481 (watch a parent folder and offer new repos)