fix: improve status detection for in-progress and closed workspaces

[milldr]

What

• Fix in-progress check to detect branches with commits ahead of the default branch (not just unpushed commits or dirty working tree)
• Fix closed check to guard against false positives from stale merged PRs with the same branch name
• Implement skip field in status spec so repos on the default branch are excluded from status aggregation
• Fix Fetch() to create origin/main and origin/HEAD tracking refs in bare-clone worktrees so status checks can reliably compare against the default branch

Why

Two bugs in flow status:

1. Workspaces with pushed branches (commits ahead of main, clean working tree, no PR yet) showed as open instead of in-progress — the check only compared local HEAD vs remote tracking branch, missing the "branch diverged from default" case
2. Workspaces showed as closed when they had active uncommitted work, because a stale merged PR with the same branch name existed — the check didn't verify the branch had no remaining divergence from the default branch

Additionally, the skip field was defined in ~/.flow/status.yaml but never parsed or evaluated, so repos tracking the default branch (e.g. main) polluted workspace status aggregation.

Ref

Bare-clone worktrees don't create refs/remotes/origin/* by default, so origin/main was unavailable for comparisons. The fix creates a targeted tracking ref for just the default branch during fetch (avoiding case-conflict errors on macOS from repos with case-colliding branch names).