refactor: improve rebase detection and add post-rebase workflow

[kostyay]

• Extract CommandUI interface to decouple UI notifications from full CommandContext
• Simplify default branch detection by looping over branch candidates instead of duplicate checks
• Add --autostash flag to preserve uncommitted changes during rebase
• Introduce showRebaseSummary() to display commit count and short log after successful rebase
• Introduce offerForcePush() to safely prompt for force-push-with-lease after history rewrite
• Expand conflict resolution guidance with safety protocol preventing dangerous operations (force push without lease, credential staging, config changes)
• Improve code readability by collapsing single-statement if blocks