Route slog and sideband progress through the live ticker

When --verbose and --progress were combined, two stderr streams fought for the same line: slog's INFO lines and go-git's sideband server progress ("Enumerating objects: ...", "Compressing objects: 36% (98/271)") collided mid-character with the in-place ticker frame. Verbose+progress was effectively unusable.

Add a sessionStderr writer that routes through progressReporter. notify() (clearing the frame, printing the message, re-arming the redraw on the next tick) when a reporter is attached to the session, and falls through to os.Stderr otherwise. Both '\n' and '\r' are treated as line ends so sideband percentage updates each become their own line above the ticker rather than overlapping with it.

Plumb the sink in two places: as the slog handler's writer in newSession when Verbose is set, and as Conn.ProgressOut on every gitproto connection so progressSink in fetch.go/push.go writes there instead of os.Stderr. The ticker's own '\r' in-place writes remain the only thing on the live line.