feat: add global exception handling

[jovnc]

Overview

Hide unexpected exception stack traces from users.

• Catch unexpected exceptions at CLI startup and in the REPL so users see a short ERROR line instead of a Python traceback
• Log the full stack to .gitmastery/gitmastery.log when a project root exists
• All other expected errors remain unchanged

Test Plan

• Terminal shows one ERROR line (no traceback), exit code 1, on unexpected error
• REPL should not exit the session on unexpecteed error
• Confirm .gitmastery/gitmastery.log has the traceback if inside gitmastery folder

No e2e test is needed, not part of critical path to test.