[hub] Add push preflight summary before committing project changes

[S1mplector]

Module: Hub / Tooling

Goal:
Before Engine Hub commits or pushes project files, show a concise preflight summary so users understand what will happen.

Acceptance criteria:

• Summary includes branch, remote URL, ahead/behind count, changed file count, and credential check result.
• User can cancel before commit if remote state cannot be fetched.
• Hub avoids making repeated commits when push authentication is known to be unavailable.
• Tests cover preflight result formatting and failure handling.

Notes:
This should make project sync safer for non-technical users.