Fog of war reveals the ground around the bot through rock (t17)

Also: board rule to write tasks from chat in our own words, task
descriptions rewritten that way, and a note on lost web-board descriptions.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/systems/fog.md

@@ -13,6 +13,10 @@ Every four ticks, 256 rays from the bot's centre reveal up to 48 grains. The las
 grains fade out. Solid and powder stop sight after a four-grain fade into their face;
 air, gas and liquid pass sight. Bars and machine voids do not block it.
 
+Each look also senses the ground around the bot through rock: every fog cell within
+`FOG_SENSE` (20 grains) of the bot is revealed, fully clear out to 16 grains and fading to
+nothing at 20, so the player can see ore and caves next to their tunnel and choose where to dig.
+
 ## Saving
 
 The fog buffer is saved and exported with the other typed arrays, without a save version