feat: add a recent satellite imagery tool

Add get_recent_imagery. It searches the recent Harmonized Landsat and
Sentinel-2 catalog for an area, picks the image the layer would show
with the layer's own ranking, and returns it with its date, sensor and
cloud cover, falling back to the VIIRS daily overview.

docs/TOOLS.md

@@ -129,6 +129,7 @@ public feeds the sources already use.
 | `get_weather` | `weather` | Current conditions at a place or point |
 | `get_weather_map` | `weatherMaps` | The latest NOAA radar, satellite or lightning map image over an area |
 | `get_wind` | `wind` | GFS or IFS model wind 10 m above ground at a location |
+| `get_recent_imagery` | `imagery` | The most recent clear Landsat/Sentinel-2 image of an area (VIIRS fallback) |
 | `get_regional_brief` | `regional` | What and where a location is, its weather and recent headlines |
 | `get_cyclones` | `cyclones` | Active NHC/CPHC tropical cyclones, optionally in an area |
 | `get_fire_perimeters` | `perimeters` | Mapped WFIGS wildfire perimeters in an area, largest first |