fix: shell out for git remotes when go-git can't read the repo

[Di7design]

gitutil.ExtractRemotes fails in two cases that come up when running the CLI from a git worktree:

• The repo has extensions.worktreeConfig = true (set automatically when any worktree gets a per-worktree config). go-git rejects this extension, so PlainOpenWithOptions errors out.
• Inside a git worktree, .git points to the worktree's gitdir but remote config lives in the common gitdir's config. go-git opens the worktree's gitdir and silently reports zero remotes.

rill devtool start is gated on this check and fails to start in both cases.

• Fall back to git remote -v (via shell) when go-git can't open the repo or returns no remotes.
• git is already a hard dependency for gitutil.CloneRepo, so this adds no new prerequisite.

Checklist:

• Covered by tests
• Ran it and it works as intended
• Reviewed the diff before requesting a review
• Checked for unhandled edge cases
• Linked the issues it closes
• Checked if the docs need to be updated. If so, create a separate Linear DOCS issue
• Intend to cherry-pick into the release branch
• I'm proud of this work!

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Developed in collaboration with Claude Code