Function is_completed 

pub fn is_completed(spec_id: &str) -> Result<bool>

Check if a spec is completed (ready for finalization).

A spec is considered completed if:

• Status is in_progress
• All acceptance criteria checkboxes are checked ([x])
• Worktree is clean (no uncommitted changes including untracked files)

Edge cases:

• Spec with no acceptance criteria: Treated as completed if worktree clean
• Spec already finalized: Returns false (status not in_progress)

Errors

Returns an error if:

• Spec file is unreadable
• Worktree is inaccessible (git status fails)