If I put a static mirror on the front of our house, even on an angled mount, it would only send light to the garden for a short period each day. To be actually useful, the mirror would need to be dynamic, tracking the sun: a heliostat. Since the movement of the sun is well understood you can do this without any sensors, just calibration.

With a bit of looking I found two companies selling heliostats, though I think the main residential use is bringing more light in through windows and not gardening. Possibly building something would be fun, but I'd be nervous about wind loads and breakage filling our own front garden with shards. Has anyone done a similar project?

[1] Boston's latitude is 42°, so the peak height of the sun on the equinox is 48° (90°-42°). The solstices are ± 23° (axial tilt) so at solar noon on the summer solstice it gets up to 71°. The house is ~40ft tall so its shadow is always > ~13ft (40/tan(71°)).